Ace, Fearless Rebel

If you choose no target for Ace's Nitro-9 ability, you will put a +1/+1 counter on it. However, if you choose a target and it is an illegal target as the ability tries to resolve, you won't put a +1/+1 counter on Ace. In either case, no fight will happen and no creature will deal or be dealt damage.

The Doctor's companion ability allows you to have two commanders if one has the ability and the other is a legendary creature that is a Time Lord Doctor and has no other creature types. Creatures with the changeling ability, for example, can't be a second commander this way.

If your Commander deck has two commanders, you can include only cards whose own color identities are also found in your commanders' combined color identities.

Although Doctor's companion is a new variant of the partner ability, the rules for partner have not otherwise changed. Notably, Time Lord Doctors and cards with Doctor's companion do not interact with cards which have another partner ability.

An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.