Q
Having lost the battle last turn, with ★3 in my power charger, I set a power cost ★5 character card and When he had a head
in the set zone.
Can my new ★5 character card attack?
A
Yes, it can.
When he had a head's effect reduces the power cost the character card set with it needs to attack by ★2, so it can attack even with only ★3 in your power charger.
However, When he had a head's effect only lasts that turn, so if your power charger is still ★3 next turn and the power cost ★5 character card is still in the battle zone, it can't attack.