Yin

Yin, A.K.A. Experiment 501, is an illegal genetic experiment created by Jumba Jookiba. She is designed to attack with blasts of water from her tentacles. Her one true place is creating a new island with Yang.

Personality
An experiment of few words, Yin was shown to be cocky, short-tempered and reckless before her rehabilitation. She is single-minded and will focus only on what's right in front of her, paying little heed to anything else going on around her. This clearly demonstrates an overconfidence in her own powers, as Yin considers them more than adequate to handle any threat. She is shown to not really care about other people's surroundings when using her squirting powers. However, she would defend her friends and ʻohana loyally, alongside Yang.

Lilo & Stitch: The Series
Experiment 501 was the 501st genetic experiment created by Jumba with Hämsterviel's funding. She was designed to spray massive blasts of water. 501 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.

All of the experiment pods were released and scattered across the island of Kauai, with 501's pod landing in a palm tree.

501's pod fell out of the tree and into a resident's pool below, activating the experiment. After the woman relaxing peacefully in the pool fled in fear, 501 then absorbed the pool's water and, armed, set out to cause chaos.

501 traveled to the beach, where she was soon discovered by a little boy. When the boy squirted 501 with a super-soaker, the latter responded by attacking him and the rest of the beach with blasts of surging water.

501 later attacked Kokaua Town. Stitch tied 501's tentacles together to render her defenseless, but this only caused her pain and her stored water to gush out and cause a flash flood.

Stitch's incompetence caused a rift between him and Lilo, and the two insisted that they each could catch an experiment without the other. Stitch then joined up with Jumba, and Lilo joined up with Pleakley to find out.

501 was later chased by Lilo and Pleakley through a jungle, but the two failed to catch the former. While draining a pond to arm herself, Experiments 501 and 502 met for the first time. Jumba erroneously stated that if they both touched, Earth would likely be destroyed. However, they were ambushed and captured in containment orbs by Gantu before they could make physical contact.

While returning to his ship with the two captured experiments, Gantu fell for a trap laid by Experiment 625, allowing 501 and 502 to escape.

When Stitch later tracked 501 down to a backyard, the latter used a running tap to gain an infinite supply of water to use as ammo. However, her ammo ran out when Lilo turned the tap off, allowing Stitch to easily capture 501 in a container.

501 and 502 were then rehabilitated, named Yin and Yang, and given a one true place using their combined powers to create a whole new island.

Yin reappeared in "Elastico" as one of the audience members for Lilo's hula dance.

In "Houdini", Yin was one of the audience members for Stitch's bungled magic act.

In "Finder",  Yin was seen playing poker with Stitch, Yang, Richter and Cannonball.

In "Dupe", Yin was one of the experiments that came to Lilo and Stitch's slumber party wearing a bathrobe and matching nightcap. However, she was forced by Pleakley to leave due to her destructive behavior.

In "Angel", Yin was one of the experiments that Angel reverted to evil using her song. Later, Yin and the other depraved experiments attacked Stitch aboard a ship, but Angel turned them back to good before they could finish Stitch off. Working together, the experiments were then able to successfully escape the ship with Stitch.

Yin reappeared in "Slugger" as part of Lilo's softball team.

In "Shoe", Yin was one of the experiments that helped turn Jumba's ship into a hotel.

Yin was one of the experiments in "Checkers" that joined Stitch in a rebellion against Gantu. Earlier in the episode, she, along with Digger and Backhoe, helped Lilo make mud.

In "Ace", when Mortalgax arrived on Earth to decide whether or not Jumba was still evil, Yin and Yang posed as mutant creations as part of a hoax to fool the head of E.G.O. industries.

Leroy & Stitch
In the film's opening, Yin can be seen waving hello to Lilo, along with Yang and Cannonball. She was also seen later in the movie creating a new island with Yang as Lilo was making her rounds with Scrump to make sure all the experiments were happy in the one true place they belonged.

Later, the first 624 experiments, including Yin and Yang, were rounded up by Leroy and taken to a stadium to be destroyed. However, Lilo, Stitch, Jumba, Pleakley, Reuben and Gantu arrived before the experiments could be destroyed.

Yin and Yang participated in the following battle between the experiments and the Leroy clones by using their combined powers to encase Leroys in rock. Yin also sprayed water at Leroys until a Leroy clone completely tied her up, leaving her powerless and Yang to fight on his own.

However, the Leroys soon gained the upper hand in the battle, but were defeated when Lilo, Stitch, Reuben and several other experiments performed the song "Aloha `Oe", which caused the Leroy army to shut down due to the original Leroy's fail-safe.

Trivia

 * Although Yin is actually a female, Lilo erroneously referred to Yin as a male.
 * Her name is derived from the concept of Yin and Yang; in this case her name stems from the force of Yin, which is associated with femininity, water and cold, fitting her nature.
 * Though Yin apparently has no mouth, a mouth is seen on her in "Angel" when she and other experiments carry Stitch out of a ship. A mouth also appears on Yin in a few scenes of "Slugger", especially during Lilo and Mertle's argument over the game rematch.
 * Yin's pod color is blue.
 * Her number was also given to Richter (in "Ace"), most likely due to Jumba's untidy database.
 * Yin is described by the experiment computer screen as, "Experiment 501. Primary function: Water sprayer."
 * Yin's head and body appear in the online game Jumba's Lab.