Jake the Dog is the second main character in Adventure Time and Finn the Human’s best buddy for life! Jake’s main power is stretching himself to amazing sizes and shapes, which come in useful pretty much all of the time.
Jake is Finn’s non-legal half brother (not his “pet”) since Finn was brought up by none other than Jake’s parents Joshua and Margaret. It is revealed in “Crystals have Power” that Jake has another brother, Jermaine. In his childhood, Jake beat up Jermaine, and was scared that his brother was dead. Jake and Jermaine must have some kind of mental connection, because later on in the episode, Jake sees with his mind a vision of Jermaine telling Jake that it was fine, and that he was still alive.
Jake’s wife/girlfriend is Lady Rainicorn, and speaks to her in Korean, but sometimes Finn and Jake put a translator on Lady, and she sounds like an old man. Jake typically likes “babes” but Lady Rainicorn is Jake’s Number One. It is unknown whether Lady is Jake’s Girlfriend or his wife.
In “Freak City”, Jake tells Finn that he had secretly always wanted to become a giant foot, and when Finn asks why, Jake unhelpfully tells him that he would find out when he was older. In “Storytelling” it is revealed that Jake can be cured from Illness if Finn would tell Jake a story with Excitement, Romance, Suspense, and a happy ending. Jake also has a tendency to do things without thinking, like stealing boots in “City of Thieves”, and stealing doughnuts in “The Witch’s Garden”. Jake obviously has occasional arguments with Finn, like in “The Eyes” (whether a creepy horse was a blessing, fitting them together, or a sin, tearing them apart) and in “The Pods” (whether a pod filled with ice cream was poisonous or not) but besides that they are cool and tend to have wild fun with each other.
Jake's Stretchy Powers allow him to do all sorts of things such as Key Hand, although in "The Limit", Jake stretches way too far. The Stretchy Powers also allow him to grow big or shrink small. In "Mystery Train" Jake stretches himself and puts on some paint to disguise himself, resembling an entirely new person, The Conductor. In "The Chamber of Frozen Blades" Jake stretches himself into a slightly irregular Ice King. In "The Witch's Garden" Jake reveals that he got his powers while rolling in a mud puddle.
Jake is terrified of Marceline, and in "Evicted!" Finn and Jake abandoned their house rather than fight Marceline because Jake was so terrified of vampires.