Serra Redeemer

Creature — Angel Soldier

Flying

Whenever another creature you control with power 2 or less enters, put two +1/+1 counters on that creature.

Serra Redeemer's second ability checks the power of a creature only at the moment it enters the battlefield. If it enters with counters, those counters are included. If that creature's power is less than 2 when it enters the battlefield but becomes greater than 2 after the ability triggers, you will still put two counters on it.