From Archon Arcana - The KeyForge Wiki
Each creature has a power value, which is the number with red background to the left of the creature’s name. A creature‘s power can be modified by card abilities or by +1 Power counters.
In a fight, a creature deals damage equal to its power to the opposing creature. If a creature has taken damage equal to its power, it is destroyed.
Damage on a creature does not reduce its power. Related Topics: Damage, Destroyed
—KeyForge Master Rulebook v17 April 2023