Board Thread:Theories/@comment-27441457-20151225042055/@comment-25322628-20160609034253

Reiner's hardening is automatic, but only when he transforms. His armor is likely dense bone.

Annie's crystallization is at will, and is made of a diamond material, harder than Reiner's, so she's likely stronger than him.

But each shifter has their own strength and specialty in a given situation.