Ayra +def -res or neutral?

That's up to you. Do you plan on, or see yourself, getting any more Ayra's for merging?

As far as which to keep, that depends on builds, really. If you're giving her Distant Counter, then it's probably best to keep Neutral so she won't take too much damage from magic attacks, but if you're going for a build that's not Distant Counter related, then +Def -Res is the best one since it increases her survivability.