|First appearance in comic||
In the comic
Mordecai first appears in "Hatchetman". It is revealed via flashback that shortly before the timeframe of the comic, Mordecai, Nico and Serafine are involved in a situation that creates a "high-profile" mess and leaves Mordecai with blood in his fur and wielding a hatchet. Serafine suggests that they turn to the pig farmers for assistance, and Nico notes that the pigs "eat anything".
In "Muckamuck" it is revealed that the target of Mordecai's attentions is contained in a large sack, though the contents remain unclear. "Vaudevillian" later establishes that Mordecai was killing multiple people.
Mordecai appears with Asa at a speakeasy in the Hotel Maribel in "Cutthroat", inquiring as to Nico and Serafine's origins. Because he is working with them, Mordecai feels he should know something about them, but he has difficulties understanding them.
In "Vaudevillian", Mordecai and Asa discuss their agenda further, but Asa claims that he does not have any additional information for Mordecai; he is essentially a "messenger". They proceed to meet with Avril, one of the pig farmers, in Asa's office. After Avril describes the destruction of the pig farm by Rocky, and points out that Marigold has been using the farm to dispose of bodies but has not paid the farmers, Asa agrees to lend them, "some very indirect assistance". As an apparent aside, he notes that Mordecai has recently procured unspecified items for Asa and his associates.
Mordecai is next seen in "Demurral", sitting in Asa's office while Asa is on the telephone with Mitzi. She suggests that Mordecai stole guns from Lackadaisy's storeroom and Asa provided the pig farmers with the guns knowing they would be used to attack Lackadaisy. Asa claims not to have known the pig farmers would be attacking Lackadaisy and advises Mitzi to consider closing Lackadaisy "before someone gets hurt". Apparently uncomfortable talking over the phone, Asa ends the call after offering to treat Mitzi to lunch at the Mill. He proceeds to tell Mordecai that he was hoping the pig farmers would deal with Lackadaisy, and tells Mordecai to accompany him to his meeting with Mitzi in order to, "help drive the point home".
In "Luncheon", Asa and Mordecai are at the Bevo Mill discussing their upcoming meeting with Mitzi, with Mordecai expressing mixed feelings about his presence there, as he feels it is a "distraction" from other work he has been assigned. Asa reiterates that he would prefer not to be there as well, but that it is "necessary" for him to meet with her, and advises Mordecai to avoid positions in management. Upon Mitzi's arrival she indirectly refers to Mordecai's presence as, "a helping of salt to rub in my wounds". In "Heartstrings", Mordecai is greeted excitedly by Rocky, who refers to him as "Ol'Serious Face" and states that he has heard stories about Mordecai. He is then confronted by Mitzi, who accuses him directly of looting Lackadaisy's storeroom and almost getting Viktor killed. Mordecai notes that Viktor should not have been involved, that in fact Mordecai reasoned with Viktor to retire before Mordecai left Lackadaisy himself. Though Mitzi points out that Mordecai's form of reasoning involved kneecapping Viktor, Mordecai responds that that is, "how one reasons with Viktor". While Mordecai does not respond to Mizi's allegations, Asa acknowledges their role in disarming Lackadaisy. After Asa states in "Loggerheads" that he warned Atlas that he was getting in over his head; Mordecai gives him a sharp look before excusing himself.
"Interloper" reveals that Mordecai has sat down in Mitzi's car; when she gets in he asks to speak privately with her. In "Hamstring" Mordecai asks Mitzi whether she believes that Asa warned Atlas; she replies that Asa probably just said that in order to intimidate her. While granting the possibility, Mordecai confesses to Mitzi that he wonders whether there might be some truth to it; according to him Marigold has been concerned about something and dealing with it aggressively, though he does not know the root cause. Mitzi notes that, in any case, Asa could not know the circumstances surrounding Atlas's death unless Mordecai told Asa something, but Mordecai states that he would not do such a thing. "Hamstring" includes a flashback of Mordecai with a gun in his hand standing opposite Mitzi, though it is unclear whether the gun is being passed from Mitzi to Mordecai or vice-versa. Mitzi expresses her belief that Mordecai is overanalyzing the situation because Atlas's name was brought up in "Monomania", then asks Mordecai what he wants from her. It is made clear that Mordecai holds Mitzi responsible for Atlas's death to a degree, and he goes on to advise her that she has brought her current problems on herself and that she can bring nothing but disgrace to Lackadaisy. Not wanting to be seen by approaching police officers, Mordecai exits the car after advising Mitzi to stay out of Marigold's way.
There is a picture of a young Mordecai with his family in "Preview 0023". He is also shown as a child with glasses in "Tiny Narratives (Q&A)", in which it is made clear that he grew up in poor living conditions. Tracy notes that he was "nerdy, sullen and desperately in need of personal space, but not profoundly maladjusted".
"Sordid Sniffles" occurs shortly after Mordecai has shot and killed Sniffles in a car. According to Viktor he was a good driver who suffered from a sinus problem. Mordecai retorts that Sniffles's sinus problems were displeasing and he smelled a little bit like mayonnaise.
In "Valentino" Mordecai notices a woman making eyes at him, mistaking it for rudeness. Viktor corrects him and suggests that he give her a Valentino-like look in return. Mordecai's attempt is...unusual.
While driving together with The Poor Schmuck bound and gagged in the backseat of the car in "Quick-fix", Viktor and Mordecai get into an argument regarding the vehicle's symmetry, to the point that Viktor forgets he should be focusing on driving. The problem is resolved by removing The Schmuck from the equation.
The aftermath of a confrontation between Viktor and Mordecai and at least five other individuals is shown in "Massacre". Viktor and Mordecai are the only ones who are conscious. Though Mordecai is initially concerned about what emotional state he should be in following the fight, he is ultimately far more alarmed about his missing cufflinks.
"Scrapbook" features a wanted poster for Mordecai. The poster indicates that he has no known prior convictions, but is suspected of involvement in several murders, including two police officers. A reward of $1500 is offered in exchange for information leading to Mordecai's arrest. There is also a picture of Mordecai with Atlas.
In "Tannenbaum", it is shown that Mordecai once gave Viktor a red tie for the holidays...he claimed it was because he was embarrassed to be seen with Viktor and hoped the tie would inspire Viktor to make better clothing choices.
In "Wassail", Viktor and Mordecai have been volunteered by an unknown party to provide food for a party. In the midst of making cranberry sauce using a particularly splatter-worthy recipe provided by Viktor, they are interrupted by carollers. Afterwards, Mordecai notes that he would like the name of the party-planner so that he can share his recipe for murder...pie.
Mordecai appears in a New Year's picture taken in Lackadaisy in 1926, shown in "New Year".
In a rare display of humor on his part, Viktor wears Mordecai's glasses in "Disguise". Mordecai appears less than amused.
Appearance and personality
Mordecai was born on March 28, 1899 in New York City, making him 28 years old at the time of the comic. According to a wanted poster seen in "Scrapbook", Mordecai is 25-35 years old, stands 5'9"-5'10" and weighs 140-150 lbs. He has green eyes and speaks with a slight Northeastern accent. He has used Elijah Metzger as an alias. No real name is provided. Mordecai is traditionally a Jewish name, and in "Tannenbaum", Mordecai wishes Viktor a Chag Sameach, reinforcing the notion that he is Jewish. According to Tracy, Mordecai completed his primary education. He prefers to cook his own food, "in which the major ingredients are separate or at easily identified, kosher, and containing nothing that smells suspiciously like mayonnaise."
"Tiny Narratives (Q&A)" establishes that Mordecai grew up in poor living conditions. This contributed to some of his eccentricities regarding hygiene and dressing well-
Being particular has been a near life-long inclination, but life experiences seem to have honed (or exacerbated) his focus on specific things. He didn’t have a big brother to inherit clothing from, but his ill-fitted, awkward clothes were either his father’s before they were his, or they were simply acquired second-hand. Atlas taught him a little something about the importance of presentation, though, and provided him the means to have tailored clothing for the first time in his life. Dressing sharply was henceforth an imperative.—Tracy, 
Tracy additionally notes that in his youth, Mordecai was "nerdy, sullen and desperately in need of personal space, but not profoundly maladjusted". When Mordecai began working for Lackadaisy he made the occasional attempt to fit in with the others, though he was never entirely successful.
Mordecai worked for Lackadaisy, and a picture seen in "Rendezvous" shows Mordecai with Atlas, Viktor and others. Mitzi notes in "Heartstrings" that Mordecai and Viktor worked together for years, though by the timeframe of the comic Mordecai is employed by Asa.
Perhaps Mordecai's most distinctive feature is the pince-nez that he wears, and has worn since he was a child. According to Tracy, Mordecai experiences sporadic double vision, and his unadjusted vision is somewhere between 20/60 and 20/80 on the Snellen chart. Though Mordecai normally wears a suit and tie, he has been seen stripped down to his underwear while performing bloodier tasks.
Mordecai does not wish to be seen by police officers, probably due to his criminal record.
Mordecai has few qualms about violence. In his first appearance in "Hatchetman", he is shown wielding a hatchet and with blood matting his fur, though it is unclear what has caused this condition. "Vaudevillian" makes it explicit that Mordecai was killing people, and "Hamstring" establishes that Mordecai did not know the motive behind his orders to kill them. Beyond physical violence, Mordecai appears to be prone to emotional outbursts over matters such as cufflinks and symmetry, as shown in "Quick-fix" and "Massacre". Beyond his knack for violence, Mordecai may have some skill as a safecracker, as shown in "Preview 0021".
Mordecai appears to have very little understanding or a sense of humor.  When he does make jokes, he seems to do so in a very dry manner. In line with these traits, he tends to speak in a very formal manner. Mitzi notes that Mordecai, "always (has) to do things the creepy way". Additionally, as illustrated in "Valentino", Mordecai has difficulty recognizing when a woman is interested in him, and greater difficulty reciprocating. Though he rarely smiles, Mordecai can be seen to do so in "Tannenbaum" as he presents Viktor with a tie for the holidays...though the smile quickly dissipates as Mordecai reveals that the tie is an attempt to get Viktor to more carefully consider his wardrobe.
In response to reader questions regarding his sexuality, Tracy has noted that, "whether it's simply his natural state of being or it's self-imposed, his 'present' existence is asexual".
In regards to whether Mordecai has any hobbies, Tracy noted that, "Mordecai is generally working even when he’s not officially working. There’s almost always something to prepare for, and when there’s not, he keeps his aim practiced, he buys and sells stocks, juggles money between accounts, sends mail, reads (for the applicable knowledge more than the enjoyment..he says), cleans, paces and sips tea". 
Mordecai appears to be subordinate to Asa. In "Vaudevillian", Asa refers to Mordecai as his shadow. When Mordecai asks Asa for more information regarding their agenda, Asa claims that he does not know any more than Mordecai does. Asa is also surprised that Mordecai fell for "the hatchet joke", implying that he was not aware that Mordecai was not from the St. Louis area.
In "Loggerheads", Mordecai looks at Asa sharply when the latter claims that he warned Atlas that he was getting in over his head, then abruptly excuses himself from their lunch meeting with Mitzi and associates. Mordecai consequently refers to Asa's "gibbering" in "Hamstring", suggesting he does not have the highest opinion of his current employer.
When Mordecai began working for Lackadaisy, Atlas provided him with tailored clothing for the first time, and taught him the importance of presentation. Mordecai has dressed sharply ever since.
In the present, Mordecai appears to have an atypical degree of interest in matters relating to Atlas, though it is unclear whether this is due to Atlas being his former employer, or whether Mordecai has other motivations. In "Monomania", Mitzi accuses Mordecai of overanalyzing a situation solely because Atlas was mentioned. "Scrapbook" includes a picture of Atlas and Mordecai together.
"Rendezvous" is the first episode to indicate Mordecai's previous allegiance; it is revealed that there is a picture in Little Daisy showing Mordecai, Atlas, Viktor and other individuals together in the cafe. In "Portraiture", while speaking to Atlas's portrait Mitzi refers to Mordecai as "that wayward golden boy of yours". "Heartstrings" confirms that prior to Atlas's death Mordecai worked for him, and Mordecai shoots Asa a look when Asa claims to have warned Atlas that he was getting in over his head. In "Hamstring", Mordecai asks Mitzi whether she believes that Asa in fact warned Atlas, though she does not believe Asa, Mordecai notes that Asa may have been telling the truth. Mordecai does have information regarding Atlas's death, though he claims that Mitzi is the only other person he has shared it with.
Mordecai seems to feel little beyond contempt for Mitzi. In "Monomania", he states that he states that he considers her partially responsible for Atlas's death, and goes on to state that she brought her current problems on herself, and can bring nothing but disgrace to Lackadaisy.
In "Demurral" it is made clear that Mitzi knows Mordecai, and specifically is aware that he works for Asa. It is further made clear that their relationship is not amicable, as in "Luncheon", Mitzi refers to Mordecai's presence during her meeting with Asa as, "a helping of salt to rub in my wounds".
"Hamstring" establishes that Mordecai and Mitzi both have information about Atlas's death that they have not shared with others. Additionally there is a flashback that shows them exchanging a gun, though it is unclear whether this is related to Atlas's death.
Nico and Serafine
Mordecai has been working with Nico and Serafine. They appear to enjoy taking advantage of his difficulties with humor. In "Cutthroat" he asks Asa how he found them, noting that he has trouble understanding them but wishes to know more about them if he is to continue working with them.
Mordecai is familiar with Rocky from before the former left Lackadaisy. In "Heartstrings", Mordecai recognizes Rocky as being a member of the band. Rocky calls Mordecai, "Ol'Serious Face" and claims to have heard stories about him. Mordecai is less than impressed by Rocky.
Rocky asks Mitzi whether he should confront Mordecai after he notices that her conversation with him has brought her to tears in "Haggersnash", but Mitzi tells him to keep his distance.
"Sordid Sniffles", "Valentino", "Quick-fix" and "Massacre", set before the timeframe of the comic, depict Viktor and Mordecai working together. Though they appeared to have rather passionate debates, matters always resolved themselves amicably...at least for the two of them.
In "Heartstrings", Mitzi notes that Viktor and Mordecai worked together for years, and Mordecai's disarmament of Lackadaisy almost got Viktor killed. Mordecai notes that Viktor should not have been involved, that in fact Mordecai reasoned with Viktor to retire before Mordecai left Lackadaisy himself. Though Mitzi points out that Mordecai's form of reasoning involved kneecapping Viktor, Mordecai responds that that is, "how one reasons with Viktor".
Mordecai once gave Viktor a tie as a holiday gift...though he claimed it was because he hoped it would inspire Viktor to make better fashion choices, and that he was embarrassed to be seen with Viktor.
In "Preview 0029", Tracy interviews Mordecai. He apparently is willing to dress up to iron his socks, at least when a new (electrical!) iron is involved. Tracy asks about Mordecai's drinking prowess, which leads to a flashback involving Mordecai apparently being very intoxicated and embarrassing Viktor after consuming Bunny Hugs. Mordecai is rendered speechless by this recollection. As Tracy asks further questions Mordecai elects to introduce her violently to his new iron. Before being interrupted Tracy opines that Mordecai is what one might expect were Seinfeld and Jack the Ripper to have a feline cranky baby.
"Reader Question: Viktor and Mordecai" addresses the question of what Viktor and Mordecai think in regards to readers who fancy them in a relationship. Viktor dies laughing, while Mordecai appears to be on the verge of a hemmorage.