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Recruit Vanilla, Chocolate, Cicini, Syaro, Nieve, and Nixie all within your first 9 members. If you've also fought Seana underwater, her concert will unlock when you reach a save point outside of town, go there and recruit her as well. Make sure this is still your tenth member or earlier. Then she'll speak at one of the Stele events, most likely the last one, and the achievement will unlock.