The true villain of the second season, Melvin is Marik's imaginary friend, created by Marik as a child to "give him all the hugs he ever wanted" and as a way to deal with years of abuse and live-action Disney movies. Yami is initially skeptical of Melvin's butting into the show, until Melvin proves himself more evil than Marik by revealing that he was the one who suspended Team Four Star, the creator of Dragonball Z Abridged.
Melvin's personality is more of an exaggeration of Yami Marik's insanity. Melvin constantly offers to hug people but in truth just wants to stab them (possibly also a joke that in the dub the Millennium Rod's concealed dagger is edited out). In "Requiem For A Nyeh", Melvin claims Marik invented him to deal with his loneliness, as Melvin "would give him all the hugs he'd ever need". He frequently asserts that he is controlling a person's mind despite characters denying this.
Melvin and Marik share the same habit of calling people "Binky-Boy", and he also tends to give people other bizarre nicknames, such as "whore-biscuit" for Mai, "chucklenuts" for Ishizu, "Doctor Scuzzbucket" for Marik's father Hank, and "Susan" for Joey. He is "your worst nightmare created in the flesh," and hates bad fan fiction in which people "make him Yami Yugi's long-lost brother" and no card games exist. Unlike Marik, Melvin does not have the special censor when Marik says the word "f***". When he says "f***", it gets censored the same way every other swear word gets censored.
In "Eternal Sunshine Of The Spotless Mai" when dueling Mai, he claims that his bird friend Mr. Tweetums was his "best friend [he] ever had".
In "Requiem For A Nyeh", it is revealed that Melvin is actually working for 4Kids, as he is the single most evil entity on the show. This is ironic as Marik despises 4Kids. The most reasonable explanation as to why he sides with 4Kids may come from their shared hatred of Disney since live-action Disney films were a contributing factor to Marik's childhood abuse, resulting in Melvin's conception.
In "O Mokuba, Where Art Thou?" when trying to open a locked door that uses a four digit code, Melvin tries to use the words "STAB", "KILL", "MUTILATE" and "PAIN". When they don't work, he asks why they are the only words he knows.
In "Muerte Pinata", Melvin progressively gets angrier at the Pharaoh, finally culminating the first moment he has genuinely snapped. Otherwise, he has shown calm, yet homicidal behavior in the past, never getting angry to the point of a furious rant and instead offering more violence- i.e., when Joey told him to go "Hump a stump," and he delightedly chooses burning Joey alive as a way of killing him in place of asphyxiation. Also within the episode, Melvin displays cowardice as soon as the tables have turned against him when the Pharaoh summons Obelisk before he can recover from loosing Ra/Mega Ultra Chicken, quickly asking if they can "Play another game, like rock-paper-scissors". The Pharaoh chooses Rock, Melvin chooses "Umm", but Rock beats Umm and Marik regains control. Melvin commands Marik to take the Pharaoh's power so they can rule the world together, but Marik refuses and questions how Melvin can rule over anything when he is "just an eyeball". Marik forfeits the match, leading to Melvin's demise.
Within the fanfiction LittleKuriboh has written, Melvin appears in Censored Town. Though he showed semblance of his typical personality, it is worth noting that the Melvin within the fic is a lot more serious than usual; very akin to his canon counterpart. Though it is up for debate due to Censored Towns' ambiguous canon status, Melvin apparently often takes over Marik during the night, where he tries to kill Bakura in cold blood. Marik calls these ' Dreams', and they seem to aptly traumatize him. At the end of Chapter 5, Melvin reveals he has taken over Marik and knocks Bakura out, leading to the 6th Chapter, where he threatens death and torture, commenting that his reasons for this were simply because it would hurt Marik. Due to the fact that he forces a kiss on Bakura within the fic, along with offhand comments in the series proper, such as in "Requiem For A Nyeh", ("I knew you always wanted me inside you, Bakura!") this may imply that Melvin has feelings for Bakura. This would make sense, considering Melvin is still a part of Marik's subconscious, and more outwardly honest about his emotions. Or, perhaps, he could be jealous.