Stitch

Stitch, also known as Experiment 626, is originally a Summon from Kingdom Hearts II. He is made available by finding the Ukulele Charm in a chest in Hollow Bastion after the first visit to Space Paranoids. He also appears as a party member and D-Link in Kingdom Hearts Birth by Sleep.

Stitch's true alien form, which consists of four arms, antennae and hair-like spines on his back, is shown in Kingdom Hearts Birth by Sleep. His abilities are comprised of his blasters and ukulele, and his powers as Experiment 626. The way he arrived at Hollow Bastion and whether or not his original home world, Deep Space, was destroyed is unknown at this time.

Kingdom Hearts II
Lilo and Stitch (2002)

A strange visitor from a distant planet.

Stitch is very strong for his small size and prone to mischief.

He's always ready to help his friend Sora whenever Sora needs him.

Kingdom Hearts Birth by Sleep
Stitch is set to appear in his home world, Deep Space, in the upcoming PSP title, Kingdom Hearts Birth by Sleep. Stitch appears in his true alien form when in Deep Space, known as Experiment 626, and bearing his red spacesuit. Originally created by Dr. Jumba, 626 was imprisoned on a spaceship set for arrival on a lifeless planet, under the watchful eyes of Captain Gantu, after his creator's arrest. Stitch later meets up with Terra, Aqua, and Ventus, each of the three Keyblade heroes unknowingly helping the troublesome extraterrestrial with his escape.

While unknown exactly how Terra, Ven, and Aqua contribute to Deep Space's storyline, recent scans have provided various hints. While Ventus is on board the spaceship holding Stitch captive, he aids the little alien in his escape from Gantu, together fleeing from the guards the fish-like alien summons, also teaching Stitch how to understand human speech. Terra is also shown, unintentionally aiding Stitch's escape as well. At first battling 626's "cousin," Experiment 221, Terra is also seen fighting off the blasters that are locked onto Stitch's genetic signature. On the trail of Terra and Ventus, Aqua also appears, 626 rescuing her from a currently unknown danger. The two battle Captain Gantu together, Stitch eventually stealing a red police cruiser as he did in the film, engaging its faulty hyperdrive to escape the alien police, and vanishing out of sight.

In the TGS 2009 trailer, Experiment 626 was seen showing Ven a paopu fruit-shaped object that he made from space parts, referencing the bond between friends.

D-Link
After helping Stitch out, he becomes a summon through the D-Link. He has only been shown to be a summon for Ventus but he could possibly be available for Terra and Aqua as well. His two finish moves are:

Reflect Beam: Collect the symbols that drop from defeated enemies while you’re in Dimension Link mode. When you have 0~1 symbols, you will shoot the beam. You can only collect up to two symbols that the enemies drop!!

Ohana Beat: When you’ve collected two symbols, you’ll be able to perform a stronger Finish move. He’ll play the ukulele and attack enemies with sound waves!!

Kingdom Hearts II
Stitch is one of two summons during Kingdom Hearts II that makes a difference to the storyline (besides Genie). His first appearance was when Sora, Donald and Goofy went into the corridors leading to Ansem's study; he was seen crawling above them without their notice. During the next cut scene, he dropped down onto the keyboard of the computer. Donald jumped onto the keyboard to grab him, but Stitch jumped off with a laugh. As Donald turned around, he mashes the keys, causing the Master Control Program to be alerted.

Later, in a cutscene showing the Final Fantasy characters fighting Heartless in the The Great Maw, Stitch was seen fighting alongside them, using his blasters from Lilo & Stitch.

Summon
Stitch can be summoned in a battle using the Ukulele Charm. Stitch is unique as a summon because he does not actually enter the field when summoned; he instead jumps and crawls around the HUD features, occasionally licking the screen. After the summoning scene, Sora tries to follow him, but comically hits the TV screen and returns to gameplay. Stitch stays on the HUD and does most of his actions from there.

Stitch is a powerful summon. When MP is used he will usually run over and 'lick' the MP bar, bringing it to full. He will do the same to the HP bar. He can shoot down enemy projectiles, effectively keeping Sora from damage. He will also bring out his ukulele, stunning enemies in midair and causing them to drop HP orbs. He will also randomly turn away from the screen and shoot enemies, simply causing them damage. His Limit attack is Ohana (Hawaiian for 'family'), in which Sora uses the Keyblade as a guitar while Stitch also plays on a ukulele.