Stitch's love interest and female counterpart is Angel (also known as Experiment 624). She's an illegal genetic experiment created by Jumba Jookiba.
Angel is designed to revert rehabilitated people and experiments to evil with a siren song, though it can have the opposite effect if the song is heard backwards.
Stitch (626) and 625 are immune to Angel's (624) song because they were created after her. But on many occasions, Stitch has been affected by the powers of lesser-numbered experiments; e.g., Spike (319) made him act 99% goofy, Baby-fier (151) made him into a baby, Amnesio (303) took away his memories.
Lilo & Stitch is our favorite story about an alien disguised as a dog and the little girl who teaches him to love. Check out these 11 things you didn’t know about Lilo & Stitch from Ohmy.disney.com.