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.
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.
Marshall Lee has a slim body, an average height, pointy ears and gray-blue 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.
He appears to have an interest in music like Marceline, as he's shown to own a bass guitar, and he greeted Fionna with metal subculture the sign of the horns, a popular hand sign amongst heavy metal and rock n' roll fans.
- Rebecca Sugar said that she fought for Marshall Lee to have a few lines (such as "Whoa, Fionna! You're wearing a dress? That is cra-azy, son!"), but they were cut for time. She wanted him to be voiced by Dante Basco. 
- In an interview with Pendleton Ward at Comic-Con, he stated that the next gender-bent episode would mostly focus on Marshall Lee.
- An alternate character sheet depicts him with green skin, but it was only used in one scene with a special lighting.