Mirage comes across as quiet and reserved, but she's a genuinely skilled fighter—a natural result of growing up in her father's dojo. Cliff trained at the same place, and the two spent their younger years sparring against each other constantly.
Her role shifts throughout the story. Early on in Elicoor II, she splits off from Cliff and Fayt to help out from the sidelines, but she officially joins the active combat roster after the group returns to Elicoor II following Styx, partway through disc two. Players can recruit her aboard the Diplo.
Mirage is one of the few characters to receive a major upgrade in the Director's Cut—she went from being completely non-playable to a full combat-capable party member. This is different from Adray, who didn't even appear in the original Japanese release before being added later; Mirage was always there in some form, just sidelined from actual battles until the Director's Cut came around.
Content compiled by AnimeList.moe from publicly available sources.