Body Regulation is an upgrade for Guapo, Juan's crocodile Amigo. Guapo heals himself during combat, so he can stay on the front line longer before he needs to revive.
Guapo slowly regains health while combat is going on. He goes down less often and can keep attacking FND soldiers longer.
Use Guapo in fights until he has self-revived five times. The upgrade then unlocks automatically.
Yes. It costs nothing but play time, and it makes Guapo much more reliable.