Agatsuma Zenitsu

Haste, First Strike
You may exert Agatsuma Zenitsu as it attacks.
When you do, put a +1/+1 counter on it, then it
gains double strike until the end of turn.
(An exerted creature won't untap during your next
untap step.)
"If you master one, that's cause for celebration."
—Agatsuma Zenitsu