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Jermaine is Jake's younger half-brother, the adoptive older brother of Finn, and the son of Joshua and Margaret. 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. 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 all 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.
Jermaine reappears in Jake's dream as he voyages back to Ooo in "Orb." He reveals that he taken up painting landscapes. As the dream grows darker, due to appearances by Jake's alien shapeshifter parent and Nightmare Princess' interference, Jermaine decides to take off, and jumps into one of his landscape paintings to escape.
In "Abstract," Jake and Jermaine once again encounter one another in a shared dream. Jermaine is untroubled by Jake's taking on an appearance closer to his shapeshifter parent, and reveals he's now painting abstracts. Troubled by this revelation, Jake tracks Jermaine down to where he's now living in the wasteland, in a house behind a literal wall of water with Bryce as his art dealer. Jake is initially convinced Bryce did something to get Jermaine to turn his back on painting landscapes, but Jermaine explains that he just got tired of painting the same shapes over and over, and wanted to paint things that represent how everything is constantly changing. This explanation hits home for Jake, who realizes he's been unwilling to face many issues from his past, and helps him resume his more typical dog form. Jermaine also reveals that Joshua warned him Jake might go through some changes one day, although he says Joshua didn't elaborate on this revelation. He is shown to be very calm and supportive towards Jake, and appears to have found some peace since leaving the old family home.
Jermaine reappears in "Temple of Mars," where he shows up at the Tree Fort to let Finn know about a shared dream with Jake that clued him in to the events of "Jake the Starchild," and that Jake is marooned on a planetoid with nothing but a dwindling supply of clown cake and cocktail hot dogs. He also lets Finn know that he can turn to him when he needs help. Jermaine then accompanies Finn to Mars and goes along on the journey through the temple to rescue Jake; the temple does not have any obstacles reflecting Jermaine's unresolved business (as he's dealt with most of his issues), though he does give Betty Grof the inspiration she needs to change her own past. In the end, Jermaine is reunited with Jake, and is perturbed to hear that Finn recently had head lice, as Jermaine had been in contact with Finn's head several times throughout the adventure.
In "Come Along With Me," Jermaine appears in Finn and Jake's shared dream and accompanies Jake to retrieve Finn's Vault from his subconscious to help Fern. Jermaine morphs into Lady Rainicorn as Jake approaches the Vault and confronts his nightmares, so it is unclear as to whether this was actually Jermaine sharing yet another dream with Jake or just a figment of Jake's subconscious. He is seen in the final montage painting a mural at Castle Lemongrab for an appreciative Lemongrab 3.
Jermaine resembles Jake, but has missing teeth, slight chin stubble, and prominent eyebrows. In Jake's flashback, Jermaine was wearing red boxing gloves. He also appears to be slightly flabbier than Jake. However, in "Jermaine," his teeth are perfectly aligned and he has no visible lips.
- "Crystals Have Power" (debut)
- "Jake the Dad" (holo screen)
- "Joshua and Margaret Investigations" (flashback)
- "Orb" (dream)
- "The First Investigation" (time slip)
- "Come Along With Me"
Mentioned and pictured
No, man, we're just dreamin' at the same time.
Yo, that was an accident, I knew you were just horsin' off!
Go do your Goucho Marx slapstick with them goofballs in the woods.
You're never going to taste my carne asada, Bryce.
Meanwhile I gotta stay here and watch Dad's trashy booty cause I never stretched into the sky and farted on the wings of a falcon I guess.
- According to Kent Osborne, "Jermaine's off doing his thing and he likes to paint, but he doesn't really like to travel." 
- Jesse Moynihan mentions on the Season 2 DVD Commentary for "Crystals Have Power" that he has tried to find a way to use Jermaine in several episodes, but was not successful until "Jermaine."
- Jermaine shares the same voice actor with Greg Universe from the cartoon series Steven Universe, created by former Adventure Time writer and storyboard artist Rebecca Sugar.
- The name Jermaine has an Latin origin from the name Germanus, meaning "brother".