Personality

Throughout the course of the entire Lilo and Stitch franchise, Stitch is depicted as a character that is unlikable to many people except Lilo (on rare occasions, even she falls out with him, most notably in Lilo and Stitch 2, where she cries out at him that he is "bad, and always will be bad!") due to his destructive and temperamental behavior. It is interesting to note that people who are averse to Stitch, such as Mertle or Hamsterviel, describe him as "ugly and deformed", while people with an affection (like Lilo, Jumba, and Nani) for him describe him as "cute and fluffy". This reflects the basic idea behind Stitch and also his cousins: they do not correspond to many established criteria of good physical appearance, having an extra pair of arms, misshapen heads, or other odd features.
When Stitch is in his "evil" form (which is the true alien form), he is undeniably worrying as his somewhat botched physique underlines his at the time twisted behavior; in his "good" form however ( the "dog" form, that is trademarked and the audience most often sees), Stitch's display of emotion at being rejected for his misbehavior and looks while trying to act good for Lilo are meant to make him seem more adorable to the audience. Despite his surface destructiveness, which does not go away even after he has become "good", Stitch is a complex character: according to his creator Jumba, Stitch as a destructive machine was not given a higher purpose in life. Since Stitch is trapped on an island where there are relatively few things to destroy, he has many occasions to reflect on other occupations than evil. It is implied that Stitch destroys only because that is what he is programmed to do; but since he has a personality of his own (a fact that only Lilo truly understands), he also has the choice, even will, not to do so.
Despite having superhuman strength and a high level of computer-intelligence, Stitch is emotionally fragile and has a child-like personality beneath his "monster" appearance. Lilo believes that one reason why he originally pursued his destructiveness was for similar reasons to her own naughtiness, that it was because he was emotionally scarred (for having been made as a monster by Jumba) but received little understanding or sympathy from others. Stitch becomes much more well-meaning after his betterment, but is frequently unable to control all of his impulses to antisocial actions. Fortunately for him, Lilo (and later Reuben and Angel) is always there to support him, and he returns her support willingly.
There is a common misconception from viewers that Lilo is dependent on Stitch for protection and support; if anything, it is Stitch who is more dependent on Lilo. Despite her having her own troubles to contend with, Lilo makes every effort to help Stitch in life, and as the story progresses he becomes more self-conscious and capable of the right decisions thanks to his friend. At the end of the series, Stitch has friends, family, love life and a purpose worth believing in.

Stitch!

When Lilo left for college, Stitch felt enough of a void in his life that he chose to leave Kauai and go back into space. He started reverting back to his destructive programming, but years later, he would land back on Earth on Izayoi island, off the coast of Okinawa. Soon joined by Jumba and Pleakley, they make a new life with a young girl named Yuna and Stitch seeks the magic powers of the Spirit Stone, an object that can grant any wish he wants, in this case, wanting power. But the magic only works by doing good deeds and Stitch now faces the return of Hamsterviel and a once-again disgraced Captain Gantu along with various experiment threats. Eventually, Stitch reunites with Lilo again when she, now fully grown, visits Okinawa with her identical looking daughter, though things get tricky with Gantu and Hamsterviel use Morpholemew to take advantage of the situation.

Lilo & Stitch

As revealed in Lilo & Stitch, Stitch was created by "evil genius" Dr. Jumba Jookiba, who called him "Experiment 626". Both Jumba and Stitch were captured and put on trial by the United Galactic federation.
626 is then sentenced by the Grand Councilwoman to life imprisonment on a desert asteroid and Captain Gantu (who despises him) escorts him. Stitch escapes and crash-lands in Kaua'i, Hawaii. Disguising himself as a dog to hide from his captors, 626 was adopted by a little girl named Lilo, who names him "Stitch".

Stitch is trained by Lilo to be good, using her idol, Elvis Presley, as a model for his behavior. Lilo's efforts prove to be fruitless at first, as Stitch is unable to suppress his destructive programming. Nevertheless, Lilo enjoys her new "puppy". Although at first he only wanted to use the girl as a human shield from Jumba and Agent Pleakley, whose mission is to capture the fugitive experiment, Stitch slowly develops feelings for Lilo, to the point where he saves her from Gantu. After his heroics, the Grand Councilwoman allows Stitch to serve his exile on Earth with Lilo as his warder, citing her dog adoption certificate (subsequently, Lilo seems to have become not only a best friend but also a sister figure to Stitch).

Lilo & Stitch 2: Stitch Has a Glitch

In Lilo & Stitch 2: Stitch Has a Glitch (set shortly after the first movie), Stitch is living with the exiled Jumba and Pleakley in the house Lilo shares with her older sister Nani. Although still hot-tempered and mischievous, he is reasonably well-behaved, but then begins to malfunction. Since his molecules were not fully charged after his creation, Stitch experiences periodic glitches that have him reverting back to his original destructive programming, appearing to suffer from a "fit" or "seizure". Seeing this, Jumba strives to create a proper charging chamber for Stitch. Before Lilo goes on to perform her hula, Stitch scars Lilo, hurting her. Believing himself to be too dangerous, Stitch attempts to leave Earth for a remote planet, while Lilo, Jumba, and the others desperately try to have him return so they can recharge him. When Stitch crash-lands the ship, Lilo manages to get him into the fusion chamber and she pulls the switch too late. As Stitch dies, he says his last words: "Stitch sorry.". Jumba says that they're too late and there's nothing Lilo can do. But Stitch seems to be revived by Lilo's love because she said Stitch needed her more than she needed him, he's Lilo's o'hana and she'll always love him, in accordance with her belief that love is more powerful than death.

Stitch! The Movie

In Stitch! The Movie, 626 discovers the remaining 625 genetic experiments locked in a crate, in the form of dehydrated pods. In order to rescue a kidnapped Jumba a trade between Dr. Jacques von Hämsterviel and Jumba for the experiments is organized, but it instead results in the freedom of the experiments and the capture of Lilo and Stitch. Hämsterviel then tries to clone Stitch but he was rescued by Experiment 221 ("Sparky"). Stitch then rescued Lilo and landed Hämsterviel's spacecraft back in Kaua'i, where the Grand Councilwoman waited to arrest Hämsterviel.

Lilo and Stitch: The Series

In Lilo and Stitch the series, Stitch and Lilo have to capture Jumba's experiments, turn them from evil to good, and find them a home. Along the way, he meets his match Experiment 627 who is tougher than him. Stitch eventually defeats him. He also is effected by three experiment powers: Amnesio wipes out his memory, Spike makes him goofy and Babyfier turns him into a baby. However, the powers wear off. He also meets Angel (Experiment 624) who is introduced as a love interest for Stitch. Over the course of the series, he and Lilo have also had adventures with Kim Possible, Penny Proud, Jake Long, and the kids from Recess.

Leroy and Stitch

After they have successfully captured all the remaining experiments in Leroy & Stitch, The Grand Councilwoman offered Stitch the chance to become the captain of the Galactic Armada and its new flagship, The BRB (Big Red Battleship) 9000. However, that would require Stitch to be separated from Lilo. Not wanting to make Lilo sad, he chooses to return to Kaua'i. However, Lilo tells him he should go. Before he leaves with Jumba and Pleakley, Lilo gives him a necklace with a tiki of the god Ku Tiki to give him strength. She also gives Pleakley a rock, and Jumba an Elvis record. When Gantu breaks out Dr. Hämsterviel in an action of frustration, Stitch is sent on his first mission to capture the evil rodent once more. But when he arrives at Galactic Defense Industries, Stitch faces off against a new experiment - Leroy, who is an enhanced clone of Stitch. After Leroy is named, Stitch brusts into the lab and starts to fight against Leroy. Stitch fights well and is about to beat Leroy when Pleakley shows up unexpectedly, allowing Leroy to take advantage of Stitch's lapse in concentration and beat him. He is placed in a cage and sent with Jumba and Pleakley heading towards a black hole. After they are sent towards the black hole, Leroy heads off towards earth to capture all of the other experiments for Hamsterviel. Later, Stitch, Jumba, and Pleakley escape from the black hole, and arrive on the planet Turo. There they rescue Lilo, 625 (now known as Reuben), and Gantu, and fly Pleakley's car pool van back to Earth to assist the other 624 experiments in the defeat of an army of Leroys. Now, with Leroy defeated, Stitch, Jumba and Pleakley return to Earth with Lilo, realizing family is more important than spaceships, laboratories, and prestige