I'd say Iris just because unlike Lugh, she doesn't NEED another meta girl to keep her alive like Lugh needs Tish.
and I know some people say debuffs don't matter, but if you have Artemis + Iris together (until you get Lugh,) you still have the cap of -50% DEF debuff which is nice and gives you more options for EX abilities and a different soul.
Lugh is a no go until you have Tish, I have her on DMM and she's super complicated to use without a super good grid and team. Also the worst hime in AAB like ever, not that she's bad in the slightest, just... annoying.

but ye, as Slashley says Michael is probably the best option because she's the core of every light build... makes everyone burst faster.
Either Iris or Michael, for now... inb4 you find one or the other in the x10 MTix draw then MTix the other!!!