I have used treatment on my character and I have tried to use it multiple times with the game saying that maybe I could find a friend to give it to. Scouring the wiki, there isn't anything there (to my knowledge). What characters can I give it to?
You cannot use the Treatment more than once, and as of now there are no characters you can give the Treatment too. Possibly in the future there will be a Treated Reaha route, but Savin has so much other stuff to write first, not the least his own separate game.