There isn't really much to elaborate, Marcy has solid stats, a decent grid, lvl5 and 7 techs hit all opponents, innate blue for Cureall, you get her relatively early, and before getting Rainbow equipment she can wear dresses and children's clothes to pump her already beastly evade. The only drawback to Marcy imo is that her lvl7 tech requires 40 stars rather than 35.