Border Guard

Magnitude (Whenever this creature would deal damage equal to its power, it deals an amount equal to the absolute value of its power instead.)
Whenever a creature enters the battlefield, if you control it, put a +1/+1 counter on Border Guard. Otherwise, put a -1/-1 counter on it.
Border Guard cannot attack or block unless it has an absolute power of 5 or greater. (Absolute power is its power ignoring negative signs.)