Crimson Gem Saga PSP Ending?

After get Sage key they still don't know how to open the barrier then got stop by Order of Light again but finally the rival believe in Killian then Henson know how to open the barrier. After go to that castle fight big rat enemy then i confuse right here. I mean why Gelts betray them all and is that the final boss? wait that's the ending? or do i get bad ending? Geez anyway i finish this game with boring ending