"Cujo"

"Cujo" is a corrupted gem.

Despite her mind and memories having been warped by the corruption, she's still looking everywhere for her Morganite liege.

She befriended Phantom Quartz during a mission in which he was supposed to retrieve her gem to give to the Second Team for safekeeping (unbeknownst to him, harvesting), but this is the first time he actually lets a gem go, when he sees how harmless Green Calcite is, even in this state, once she reunites with Teddy Morganite. 

"Cujo" is a nickname that Phantom Quartz gave her, due to how unwillingly rowdy and destructive she was in a urban environment.

She has the ability to change size, looking like a puppy when she's happy and going into Battle form when threatened.