Serra Redeemer

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

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Bloomburrow

Dominaria United

Flying
Whenever another creature with power 2 or less enters the battlefield under your control, 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.