Nardole, Resourceful Cyborg

Card text

{T}: Add {U} for each counter on Nardole. Spend this mana only to cast noncreature spells.

Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)

Doctor's companion (You can have two commanders if the other is the Doctor.)