As revealed in Lilo & Stitch, Stitch was created by "evil genius" Dr. Jumba Jookiba, who called him "Experiment 626", with the genes of the galaxy's strongest and most ferocious creatures being zapped with electricity to start the molecules forming and starting to solidify into the form and fully formed, but when the experiment was finished, Jumba was disappointed to see how ugly and deformed he was. Nevertheless, Jumba soon sees 626's destructive potential and begins to charge Stitch's molecules. However, before Stitch is fully charged, both he and Jumba are captured by police officers of the Galactic Federation.

626 is then sentenced by the Grand Councilwoman to life in exile on a desert asteroid and Captain Gantu escorts him. Stitch escapes and crash-lands in Kaua'i, Hawaii. Disguising himself as a dog (although he actually looks like a koala), 626 was adopted by a little girl named Lilo, who names him "Stitch".

Stitch is trained by Lilo to be good, using 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 life 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).

Bio stolen in whole from Wikipedia, links and all

In-Game evolutions:  since Lilo was adopted by Brenda Annn Turner and Teague Brennan upon her arrival, they took Stitch in too when he arrived.

Thanks to the M-Day 2:  Genetic Boogaloo plot, Stitch has regained his ability to scale walls and ceilings like a gecko

Stitch is played by Sterling his PB is Andy Samberg and his journal is Not_626