Magic given sentience.
The best way to describe Cyan the slime girl. Terra, the alchemist in the crew had accidentally made her come to life. A few years back Terra and the rest of the crew had found quite the number of gems in a tomb, thus, each of them took one. The crystal-like gem she found and took seemed to come to life after accidentally spilling liquid on it. The liquid slipped and moved around the crystal as though it was a lower life form creature. Curious and incredibly interested in whatever the fuck just happened she poured more and more liquids on it until it was a shambling mass of slime-like liquid. The slime eventually took a humanoid form, eyes, mouth, and all and acted much like a child. Over the years Cyan has been an incredibly fast learner and is just as smart if not more then the usual adult. As a part of the crew, she does what she can to help out and while Terra frowns at her 'daughter' aiding in fighting, she can fight well and easily subdue her opponents.

Oddities:
Amped Up To Fight: Being magic given life, she has the ability to use it as a weapon from what little she recalls, thus she can using lighting as a means to give her attacks or defense a shocking effect.
Love's A Magical Thing: Her libido (for whatever reason) is through the roofs compared to others and is always up do to do the lewd.
Pseudo Immortality: The only true way to kill her is by destroying the crystal inside of her. Removing magic or the liquid from her body will not do anything except temporarily hinder her.
So Hot and Cold: Being magical, she can shift her temperature at will to being boiling hot to icy cold. She's able to dissolve the liquid surrounding her core as well as freeze herself, if she see's fit.

Her core is not crystal/gem like, I /know/ but I had only now come up with this concept and I really like it... I'll be making appropriate changes in the future.