(Atsushi body is weak)

Atsushi body is weak and he can't use his ability without coughing up blood after. Atsushi body was just born weak and couldn't handle transforming into a tiger.

Lucky enough there are different ways to use abilities without atsushi transforming into a tiger and atsushi just needs to find what work for him.

but the armed detective agency doesn't know about atsushi weak body and they're confused on why he won't use his ability

(You choose what happened)