Her warriors are the last defense against the coming storm.

Melira's Keepers

Melira's Keepers can't have counters put on it.

The List

Mirrodin Besieged

Melira's Keepers can't have counters placed on it.

Damage from a source with infect has no effect on Melira's Keepers. No -1/-1 counters are placed on it, and no damage is marked on it. The damage is still dealt for purposes of effects that care about damage, such as lifelink.

If Melira's Keepers would enter with a counter on it, it simply enters without that counter.

If a creature with a counter on it becomes a copy of Melira's Keepers, those counters will remain.

Melira's Keepers can't have any kind of counters placed on it, not just -1/-1 counters.