Opal

Opal was a demibeast. She was owned by one of Blue's previous masters.

History
Opal belonged to one of Blue's old masters. She was initially named Eighmey, but the other demibeasts in the household took to calling her Opal, as Opal would always take opal jewelry when the pets would get dressed up for parties. She was her master's favorite.

At some point, Opal died or was killed, and her body was donated to Majik's Academia so students could dissect it. In Chapter 42, Blue recognizes her body as the one he and Kara are to dissect.

Personality
Opal was known for being difficult and always making trouble, though she could also be very charming. She was very kind to other demibeasts, and would go out of her way to help protect them, keep her master's attention off them, etc.

Blue
Opal and Blue served under the same master, with Opal being much older than Blue at the time.