Curses, haven't been on the boards in a while due to playing CTDS obsessively for the last few days. >.< But anyway...
Looks pretty cool. For weapons, either a dagger or a bracelet/ring that shoots orbs of magic energy would be good, I think. (Maybe if you choose the bracelet/ring, damage could be based off of magic instead of strength? But that might be overpowered...)
For a storyline reason as to how you get her, I think it should be before the DD, like I said -- perhaps there could be a dialog branch when she's about to teleport you out of the Ocean Palace. You could either give her some power from Marle's pendant (which is fully charged, presumably). However, something goes wrong since the Ocean Palace is going boom, and she's teleported somewhere else. However, during The Fated Hour chapter, you could introduce another sidequest in 12000 BC(E) where she washes up on shore, but...something happens. If you killed Janus, she stays after the quest, but if you have Janus in the party, she joins... Or something like that. :/
(Also her affinity would still be light -- when you fight the Time Devourer in Cross, its affinity is White/Light)
Also, for her skillset, Holy Light is unfortunately no longer a good choice, seeing as the Master-at-arms uses an attack with that name. :( Also, I think I need to redo my original idea a bit...
*Restore
*Heal
*Cure
*Starfall
*Guardian (casts Protect and Barrier on one ally)
*Comet
*Arise
*[Holy/Holy Light/Whatever]
The pyramid seal is also an excellent idea that Rox discussed in his thread -- perhaps two dual techs with someone else, one that casts Protect and Barrier on the party, and one that inflicts a bunch of status ailments like Stop on all enemies.
As for having Zeal's abilities...no. She likely professed in a different magic type, and the only time we actually fight her is after she's turned into Lavos's puppet, so most of her attacks likely come from him.
Or maybe I'm just a rambling idiot and my ideas suck, I don't know. :/ I suck at coding, so I can't help you at all in that regard, Justin, but I hope this comes along well! It's going at a pretty fast pace already! :D (It would be cool if someone could do a DS version of this hack...though that's likely in the far future for now)