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The army character you can only recruit from Deneb's store (chapter 4) -- you have to be in the store Deneb is running that week and she'll offer to recruit you one... if you sold her a Glass Pumpkin.
Glass Pumpkin items are dropped by small enemies (gremlins, goblins, I forget) exclusive to the Hell's Gate area. Hell's Gate is a dungeon that (IIRC) opens up at the end of Chapter 3, but you can't go inside until 4? There are goblins/gremlins outside too, but you encounter them more frequently (and I think they only drop Glass Pumpkins) inside this side-stage.