After the events of Shadow of the Colossus, Mono is revived, and (knowing it's power firsthand) begins to use the "shadow energy" inherently found in the lineage of horned boys as a means to bring Wander back to her. Like Wander before her, her physical form is distorted after handling so much evil energy. During the events in Ico, she may still be trying to revive Wander, or she may have long since abandoned that goal.