Shell

"Welcome, Pink Diamond.

Steven Universe: Ah, no, I'm Steven Universe.

Welcome, Steven Universe. I am Shell, your guide to The Reef!"

Shell is an A.I unit that resides in the Reef and is the primary antagonist of the episode "Volleyball". She attempts to heal Pink Pearl's crack by rejuvenating her and Pearl. After they fuse into Mega Pearl to escape, she then attempts to kill the newly formed fusion and Steven. They escape Shell and destroy her security system.

Personality
Coded to act as a guide for possessors of Pearls, Shell is, at first, soft-spoken and helpful to Steven, Pearl, and Pink Pearl, as they walk through the facility of the Reef in search for a means to heal Pink Pearl's cracked eye. Once Steven loses his temper, however, Shell turns against the two Pearls and acts in harsh ways to "fix" them: she locks them in an oyster to await rejuvenation. This showcases the strictness of the former Homeworld and those who programmed Shell, as she retains the characteristics of a former authoritative government. The A.I. becomes aggressive upon the formation of Mega Pearl, using her defense system to intercept and capture the fusion, but is ultimately turned off by Steven.

Abilities
Shell is programmed to be both a guide and a guardian to the Reef, being in full control of it. She can light up paths indicating the way toward a room, offer information to visitors, scan Gems with sensors, and rejuvenize rebellious Pearls — all to meet the owner's wishes.

In her position as a keeper, she can activate a defense system to attack unauthorized personnel. When it is initiated, the facility becomes weaponized: the doors are locked, and Shell utilizes laser beams, stretching ropes, and pannels to prevent any intruder from leaving. However, she can be defeated.

"Volleyball"
To be added.

Trivia

 * As an A.I., Shell can be categorized as both an object and a character.