Marshall Lee (full name: Marshall Lee the Vampire King), is the gender-swapped version of Marceline, created by the original series designer, Natasha Allegri, who is featured in "Fionna and Cake." In the episode, he is seen attending Prince Gumball's Biennial Gumball Ball. Just like Fionna and others, Marshall Lee is from Ice King's imagination, although Marceline was telling his story in "Bad Little Boy." His skin appears to burn in sunlight in the episode, like most vampires' and Marceline's. He plays a major role in "Bad Little Boy," attempting various ways to trick Fionna into admitting her love for him.
Marshall Lee presumably has the same abilities as Marceline, since he was first seen floating. Some official artwork shows him eating the color red, just like Marceline, and sucks the pink from creme puff filling, in "Bad Little Boy" he mentions "Drinking the red from your heart in one sitting." He can also sing and plays a guitar. He can turn into a bat form, like Marceline, except he keeps his hair. Like Marceline, he can also raise the dead. He also appears to have the same rapid healing that Marceline has, as when the sun started to burn him, the burn vanished as soon as Cake blocked it from touching him.
Marshall Lee has a slim body, an average height, black hair, and light gray skin. He has a matching permanent bite mark like Marceline's which shows he is a vampire. In Fionna and Cake and Bad Little Boy, he is seen wearing a red plaid shirt with gray stripes, dark blue jeans, and brownish-red and white sneakers. Like Marceline, his physical appearance is that of a typical late teenager or a young adult, but he appears to be a little shorter than her. Also like Marceline, he looks about the age of 18.
Marshall Lee describes himself as a villain. He is rowdy, endangering others with his violent, 'friendly' rough-housing. He appears to have an interest in music like Marceline, as he's shown to rap and carry around and play an electric guitar, opposed to Marceline's bass guitar. He greeted Fionna with the sign of the horns, a very popular hand sign amongst heavy metal and rock fans. He also displays Marceline's love to mess with people with elaborate jokes, but goes to a further extent than his female counterpart. However, he seems to feel remorse more quickly than her. This also might suggest that Marceline wishes she could express remorse better; she rarely apologizes to anyone. Since Marceline is telling the story at this point, this might suggest the way she thinks of herself; as a villain, although Marshall Lee is arguably the more evil of the two. Marshall seems to be more violent then Marceline, saying he 'Kills people every night," although this may be an exaggeration.
Marshall Lee and Fionna are good friends. It is unknown whether Marshall Lee has feelings for Fionna, although in "Bad Little Boy" he says briefly "Why don't you just admit it? You're in love with me." Marshall Lee is first seen briefly in the episode "Fionna and Cake," when he notices Fionna entering Prince Gumball's party and flashes her the sign of the horns, sticking his tongue out. (It is noted that this is one of the few times in the show where a character is shown with five fingers.)
Similar to Finn and Marceline's relationship, Marshall likes to mess with Fionna's head. In the second episode featuring Ice King's fan-fictions, "Bad Little Boy," he zooms into her house and steals the pastries Prince Gumball had brought for Fionna, leading her outside to investigate a noise that is probably him landing on her roof. When Fionna finds him on the roof playing his guitar, he looks up and smoothly says, "Oh. Hey, Fionna." Fionna responds by smiling and says his name in a happy, excited way. He asks her to go to Lumpy Space Prince's Party, but she is uncertain at first of whether she should go, presumably because she has Prince Gumball over for company and would have to ditch him to go. Marshall transforms to bat form and reaches out his hand for hers, saying he knows she will give in. She smiles reluctantly and takes his hand before being flown into the forest. There at the party, he is seen doing obnoxious things and teasing her, all the while trying to convince her that he is evil. He even goes as far to fake a fatal injury and tell her to admit her love for him. While doing this, Fionna yells out and tells him that in his last moments he should be serious. She mentions that he likes to flirt with her and seems to think it is funny. When he sees her emotional and crying and reveals that he was only faking, Fionna angrily punches him, then chases him while he protests and proceeds to hit him again off-screen, laughing maniacally.
Marshall often messes with Cake, but they seem to be on good terms. Although he pushed her into the bushes in the episode "Bad Little Boy" carelessly, calling her 'kitty' rather condescendingly, she seemed to recover mostly and was seen smiling after Fionna picked her up and spoke in Cake's defense. She tried to break away from Marshall Lee when he kidnapped her and flew to the cemetery, which resulted in a short struggle and caused Marshall Lee to seemingly be pierced through the chest on a spear. When Marshall Lee appeared to be hurt, Cake used her body to shield him from the sun while it rose behind them. She was also visibly relieved when he revealed that he wasn't really dying, even though he had kidnapped her moments before. This completely contrasts with the way Jake the Dog persistently attempted to kill Marceline, despite his being terribly afraid of her, in the episode "Henchman".
Marshall Lee's relationship with Prince Gumball seems to be similar to Marceline and Princess Bubblegum's rivalry toward one another. When Fionna and Gumball find Marshall Lee on the roof in "Bad Little Boy", they find Marshall was the one who had stolen Gumball's cream puffs. Marshall proceeds to drink the pink color out of a cream puff, then toss it at Gumball's head. When the puff hits him, Gumball's response is to grimace and begin climbing down from the roof, telling Fionna he is going inside and she can come with him if she would like "more polite company". It is unknown what happened to him after Fionna and Marshall Lee ditched him to go to a party. It is presumable that they have a strong dislike for one another similar to that of Marceline and Princess Bubblegum.
Marsha is supposted to be his little sister. However, their relationship is weird. In Adventure Time's version of Teen Beach Movie, Marsha accidentlay falls into Marshall's arms (when she was meant to fall into Gumball's) and falls for him, instead. At other times, she annoys Marshall like how Pinkie does to Rainbow Dash. It seems the same way with Marsh and Marceline's relationship with each other.
He seems to possess the loneliness trait Marceline has, and hides it like she does.
His guitar is made from an axe like Marceline's but is a six-stringed non-bass guitar with a blade on only one-side, unlike Marceline's axe-bass, which is a 4-string bass guitar and is double-sided.
It's unknown if he is friends with the Ice Queen, like Marceline being with Simon Petrikov/Ice King during the Mushroom War.
However, the similarties/differences may depend on who is telling the story.
Rebecca Sugar said that she wanted Marshall Lee to have a few lines in "Fionna and Cake," but they were cut due to time limitations. She wanted him to be voiced by Dante Basco.
An alternate character sheet depicts him with green skin, but it was only used in one scene with special lighting conditions.
In "Bad Little Boy," his skin is shown to appear green in the sunlight.
When Marshall Lee does the "Sign of the Horns" he has five fingers instead of the usual four.
His hairstyle is different in "Bad Little Boy" from "Fionna and Cake." It appears down rather than up, however, this could be because he was first seen in the rain, which could have caused his hair to get wet and fall down.