Gary Prince is a barista and pastry chef, who dreams of opening his own bakery one day. His concept for his bakery, Pastry Menschen, revolves around humanizing the treats he creates, and selling them accordingly (for example, two different types of treats who are dating would be sold together as a package deal). He's not royalty, like his magicverse counterpart is, but his prissiness manifests in being a bit of a clean freak. In regards to the differences between him and Princess Bubblegum, showrunner Adam Muto said, "Gary's not necessarily as authoritarian as Princess Bubblegum or as together and commanding. He's just got some of the ambition, but is a little bit more nervous." I think this describes him really, really well.

Gary is a very driven person, and likes to follow routine, plans, and perfect everything involved. He has a 64-step business plan with how he wants to get his bakery open one day, and admits that he would have fiddled with the recipe forever if he hadn't been told by a pair of rich investors that the taste was already good. He loves to talk about the Pastry Menschen's storyline and business plan, as well, but seems insecure about it. When Marshall Lee goes quit while he's infodumping, he assumes that he grew bored listening to him, and apologizes for talking too much. He also assumes that Marshall Lee is poking fun at him when they ask him to explain a certain part of the storyline to them, as well.

Not much is known about Gary's past, but some things can be inferred. For example, his insecurity about talking too much or too passionately implies that Gary may have been made fun of for this trait in the past. Furthermore, he goes to a poorer and more run-down side of town in order to pick up his ingredients, which Marshall Lee verbally notes as seemingly out-of-character for him. This could be because he was raised by a lower-class family growing up. Of course, these points are both speculations. He presumably has a fraternal twin sister, as Princess Bubblegum has a fraternal twin brother named Neddy, but she is never brought up.

Gary is a very autistic-coded character. He is socially awkward, has special interests (baking and the Pastry Menschen), infodumps (bonus points for him being insecure about said infodumping, because if that ain't an autistic mood), has an inability to read the room, likes routines and plans, and talks with hands a lot. Gary is also, of course, queer, although a specific label is not given to him at any point.

Gary has known Fionna a long time ("forever", in his words), and it's shown that she is the one who helps him dye his hair, which is red naturally. He worries for her when she disappears, and is only really reassured when she messages him back, letting him know that she is alright.