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.