Marceline is a fun-loving & playful 1,000-year-old vampire queen. Unlike a traditional vampire, Marceline does not need to drink blood to survive; rather, she consumes the color red, or other similar tints. Marceline is also an avid musician who plays an electric bass guitar, which she crafted from her family's heirloom battle-axe. During her time, she has evolved into a 'fearless daredevil', turning her into a bit of a trickster. Without food, she becomes vicious and sinister, blood.

Marceline is a slim and tall half-vampire/half-demon hybrid with light blue-gray skin, sharp crooked fangs, and very long, ankle-length midnight black hair. She also sports two marks on her neck, which are the marks of the bite that turned her into a vampire. Her red tongue is sometimes bifurcated, like a snake's, but other times looks like a normal human's. This may stem from her shapeshifting powers, allowing her to change its appearance as she pleases

Marceline has a close relationship with Finn and Jake. Marceline and Finn have many things in common with one another; in the episodes "Evicted" and "Henchman", it is stated that the reason Marceline befriended Finn is that he enjoys being himself. Although she has kissed Finn platonically on the cheek twice, Ward has noted that there is no romantic subtext between the two. On the other hand, Marceline and Jake share a more complex relationship. In her initial appearances, she derived pleasure from scaring Jake, who was terrified "of her vampire bite". In turn, Jake felt that she was evil and had managed to take over Finn's mind. Due to this, Jake attempted to kill her several times, to no avail. However, after the events of "Henchman", Jake seems to have forgotten his fear of her; in "Memory of a Memory", he even refers to her as a "friend" in one of her memories.

At the start of the series, Marceline shares a tense relationship with Princess Bubblegum. However, as the show has gone on, their relationship has been fleshed out and changed. Marceline was the first person to address Bubblegum by her first name—Bonnibel—which occurred in the episode "Go With Me". Their connections are explored further in the season three episode "What Was Missing". In the episode, Marceline's song "I'm Just Your Problem" and the dialogue between her and Bubblegum implied that they may have had some sort of relationship in the past, although the exact details surrounding the nature of the relationship were not explained. This later caused an internet controversy over Marceline's sexual orientation. However, after the events of "What Was Missing", Marceline and Bubblegum's relationship seems to have gotten better, as the two spend quality time together in "Sky Witch", "Varmints", Stakes, and "Broke His Crown" and are seen having a strong bond. In the series finale, "Come Along with Me", Marceline and Bubblegum express their romantic feelings for one another and kiss, and the episode's epilogue shows the two seemingly as a happy couple. In the episode "Obsidian" of the spin-off series Distant Lands, set some years later (Finn is shown as an adult man in the episode), Marceline and Bubblegum are depicted as living together in Marceline's cave and are in an explicitly romantic relationship with each other. The episode also confirms that they had previously been in a relationship and a break-up is shown through flashbacks.