Turquoise

Turquoise is an original fictional character.

Background
So little is known of Turquoise's background that even its age, gender, and given name are unknown. "Turquoise" (or just "Turq" for short) is likely a codename given by the Brotherhood themselves. Where they found this child, or how it came to join the Brotherhood remains a mystery; however, Turquoise seems to have bonded closely with both Wither and Sublime, almost treating them like surrogate parents.

Powers and Abilities
Turquoise is a human mutant, and is likely an Alpha, or possibly Beta-level mutant. The being's entire body is either encased, or possibly wholly made up of a blue-green stone resembling common turquoise. This inhuman physiology appears to grant a degree of invulnerability, as well as considerable superhuman strength, at the expense of agility. If Turquoise really is a child, this physical awkwardness may merely be the result of inexperience.