Halana and Alena, Partners

Legendary Creature — Human Ranger

First strike (This creature deals combat damage before creatures without first strike.)

Reach (This creature can block creatures with flying.)

At the beginning of combat on your turn, put X +1/+1 counters on another target creature you control, where X is Halana and Alena's power. That creature gains haste until end of turn.

If Halana and Alena's power is somehow negative at the time its triggered ability resolves, you will put no counters on that creature. The effect doesn't remove counters from that creature or give it -1/-1 counters.