Noon is only available to join the force if Kahn's Elbesem Orb is used on the appropriate Kyon-Shi in battle #11 (Quonus Graveyard). He's the one standing at the north shore of the lake. Killing the other Kyon-Shis makes no difference. Once the Vandal Abdark is defeated, Synbios must find Noon at the graveyard lake shore (he switches to the south one, misleading) and after talking to him, he'll join the force.