Your identity as 'a member of the Armed Detective Agency'.

[Since Dazai has already made two wishes, Watanuki will answer his questions honestly.]

The coat would have been an accurate physical representation of it, I suppose, but the cost is much, much greater than the ability to hide. And asking you to give up all of yourself is not a price that is conductive to a long life. I think I should refrain from angering a certain someone too. He can actually hurt me, if he tries. Which I suspect he might if I even consider taking a deal like that.