Marshall Lee has a slim body, an average height, black hair, has light gray skin. He has a matching permanent bite mark like Marceline's. In "Fionna and Cake," he is seen wearing a red plaid shirt with grey stripes, dark blue jeans, and brownish-red and white sneakers. Like Marceline, his physical appearance is that of a 16-18 year old.
Marshall Lee describes himself as a villain. He is rude and impolite, endangering others with his violent, 'friendly' rough-housing. He appears to have an interest in music like Marceline, as he's shown to own a guitar, and he greeted Fionna with the sign of the horns, a very popular hand sign amongst heavy metal and rock fans.
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, although he says he drinks blood in "Good Little Girl". He can also sing and play a 6-string bass . He can turn into a bat form, like Marceline, except he keeps his hair. Also like Marceline, he can raise the dead.
FionnaSimilar 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.
CakeMarshall 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.
It's unknown if he is friends with the Ice Queen, like Marceline being with Simon Petrikov/Ice King during the Mushroom War.