Jermaine is Jake's younger half-brother, the adoptive brother of Finn, and the son of Joshua and Margaret. He is the only biological son of both Joshua and Margaret due to the fact that Jake came from a Shape-shifter that poisoned Joshua and Finn being found in the woods. He is introduced in "Crystals Have Power" in one of Jake's flashbacks where they were both children. Jake hurt Jermaine in the flashback, which seems to have left an impact on Jake causing him to avoid violence as a way of solving problems.
After Jake hits his head and apparently begins a spiritual communication later in the episode, Jermaine tells him that he understands Jake did not mean to hurt him. Jermaine explains that they are able to talk in this manner because they are both dreaming at the same time (though Jake does not appear to be dreaming or otherwise affected by hitting his head).
A photo of Jermaine, alongside Joshua and Jake, is seen in Finn's memory in "Memory of a Memory". In "Jake the Dad", a hologram of his younger self is seen in Mom's manual with Finn and Jake. Although Jermaine's age is unknown, he seems to be around the same age as Jake, considering they were both babies in Margaret's manual. Joshua briefly mentions Jermaine in a videotape in "The Pit" when telling his children where he has gone searching for Kee-Oth the Blood Demon.
Jermaine's birth is depicted in "Joshua and Margaret Investigations". He is revealed to be the only biological child of Joshua and Margaret, as Jake was the result of Joshua being infected with the DNA of a shapeshifting alien and creating a dog/alien hybrid, while Finn was adopted. This makes him and Jake half-brothers due to only sharing Joshua as their biological father.
In the episode "Jermaine", Finn and Jake are inspired to go visit Jermaine after Jake encounters Jermaine in another shared dream. It turns out that Jermaine lives in Joshua and Margaret's old house, where he guards various items acquired by Joshua during his adventures from demons who want them back. Because of the demons' threat, Jermaine lives an isolated existence behind a circle of salt that keeps the demons out. After Jake uses some of the salt to make fried rice, letting one of the demons in, Jermaine attacks Jake, finally expressing his frustration that Finn and Jake, who he believes to be Joshua's "favorites", get to live an easy life while he's stuck guarding Joshua's stuff. Jake convinces Jermaine that Joshua loved them both equally, and when a "flame hand" Jermaine threw at Finn sets the house on fire after Finn slices it in half, Jermaine decides to let it burn, having realized "I built my own cage." The next day, the demons are gone (their stuff having burned up in the fire) and Jermaine departs with Bryce, a demon Joshua kept imprisoned in the basement with whom Jermaine had built a tenuous friendship.