Jake

Jake is a magical dog in Adventure Time. Finn refers to him as his friend and will often get offended if others say that Jake is his dog. His known powers from the television series consist of: the ability to change in size and shape, as well as move his guts around, which repeatedly comes in handy in their adventures. He also likes to play the viola.

Appearance
Jake is a light-orange tan dog(bulldog) with large, characteristic eyes. His appearance can change drastically due to his ability to morph. His eyes are similar to that of Lady Rainicorn's. Jake is 28 years old(or 4 years old in human years), but sometimes he acts less mature.

Personality
Jake is mainly laid-back. He often cracks jokes at serious times, but if Finn is feeling disheartened Jake always has a lecture or a song to cheer him up. He always has great advice when something bad happens, like in the episode Evicted!, where he suggests house-hunting. He is afraid of Marceline because of all of the scary stories he has heard from, "reliable sources," made up or not. He believes he'll die if he eats chocolate or fudge, because a dog can't eat that kind of food. He can be somewhat irresponsible at times, frequently leaving Finn to fight most of a battle on his own, but he always pulls through in the end. He believes that running is some sort of "leg magic." He also refers to his face as his "Bread and Butter" in the episode "Ocean of Fear", and his rear end as his "powdered donut" in "Dungeon".

Rainicorn
Rainicorn is Jake's girlfriend. On the official website in Rainicorn's description it says she is Jake's girlfriend and that they get along so well because of their common interest in the viola. At the beginning of "Slumber Party Panic", he is shown playfully chasing Rainicorn, and later in the episode he doesn't seem to mind being locked in a closet for 7 minutes with her.

This relationship is also seen in "My Two Favorite People". Also, it is shown that Jake can speak, understand, and decipher what Rainicorn says.

Finn
Finn and Jake are best friends and will go out of their way to protect each other. Jake watches over Finn using his stretchy powers to protect him, whether it is shielding him from rain or cold weather, or catching him in mid-fall. Jake seems to know about Finn's crush on Princess Bubblegum (though Finn still denies it) and even helps him try to win her over, especially in the episode "Ricardio the Heart Guy". This relationship existed from the start of the series. They often do everything together (or try to, as shown in "My Two Favorite People").

Tree Trunks
Jake and Tree Trunks share a close relationship, which is similar to a grandmother-grandson relationship. This is shown most of the time, though there was a situation in the episode "Tree Trunks" when she kissed him several times so that he would let her help Finn. Jake appeared later with lipstick marks all over his face, and he was smiling. Later, Tree Trunks tells Finn that she'll accept his apology (Finn had earlier yelled at her) if he would let her kiss his cheek. Finn was about to decline the offer when Jake says "Whoa, let her kiss your cheek, man! It's a once-in-a-lifetime opportunity!" Other than this, there are no evidences of a romantic relationship between Jake and Tree Trunks, although they remain close friends.

Others
While Finn and Rainicorn are apparently Jake's two favorite people, he has a number of other acquaintances, listed in his cell phone (see Jake's Phone)

Shelby, an earthworm and friend of Jake, lives in his viola.

In "Memories of Boom Boom Mountain", Joshua, a dog who resembles Jake (but wearing a grey fedora) and Margaret, another dog who resembles Jake (but wearing earrings and a peach hat) find the infant Finn. They were in fact confirmed to be Jake's parents in the Adventure Time Frederator Blog. It is noteworthy that both Finn and Jake call each other "brother" in the same episode. This is most likely because Finn is Jake's adoptive brother. (See Joshua And Margaret)

There is also a dolphin who has fallen in love with Jake, which Finn considers to be "the opposite of a problem."

Jake and the Squirrel are enemies, because Jake wouldn't publish the Squirrel's letter in his newspaper advice column and now the Squirrel wants revenge.

Quotes
See Jake/Quotes

Trivia

 * Jake's voice actor is John DiMaggio who has also voiced with Marcus Fenix in Gears of War 1 and 2, various Brutes in Halo 3 and Halo 3 ODST, Shnitzel from Chowder, Bender from Futurama, Rath from Ben 10: Alien Force and Ben 10: Ultimate Alien, and Rico from Penguins of Madagascar.
 * Jake has at least 23 contacts on his phone, possibly more.
 * Finn and Jake can accurately tell what time it is by looking at how the sun affects the trees.
 * In "The Witch's Garden", Jake tried to remember how he got his powers, and came up with the idea that he got his powers by rolling in a mud puddle.
 * When Pendleton Ward was asked on his formspring if the mud puddle that Jake rolled in as a pup was really where he got his powers, Pen stated that it wasn't. Jake just couldn't remember.
 * Jake writes a newspaper column titled "Begs the Question".
 * Confirmed on http://www.formspring.me/buenothebear, Joshua and Margaret ARE Jake's parents, who are dead now but raised Finn as a baby.
 * Jake's parents do not have any powers. This was found out when someone asked Pendleton Ward if Jake's parents have powers on Formspring.
 * Jake doesn't take Finn seriously when he sings.
 * Jake wears invisible pants spun from spider webs by pixies, as seen in "Donny".
 * It was implied in the episode "Freak City" that Jake has a foot fetish.
 * He does a parody of "Double Rainbow Guy" except in the promo he sees a double rainicorn "all the way."
 * Jake was confirmed to be a bulldog on Pen's Formspring.