Mono, masked explorer

Friendship (When this creature enters, choose a creature you control to be it's 'friend'.)

Whenever Mono attacks, if his 'friend' is also attacking, they both get +2/+2 until end of turn.

Betrayal (When this creature's 'friend' leaves your battlefield, transform it. It this creature's 'friend' is under an opponents control, this creature gets 2 +1/+1 counters before transforming)