I didn't say about BEXP, I say give her paragon scroll.
I didn't say Mist > Rhys, I say Mist is a better healer.
Both Mist and Rhys will get S rank in staff very early, and that all their need because SS staff is for Micaiah.(You get another SS staff very late)
About Promote, They both will be promote late anyway, and Rhys at tier 2 and 3 is identical (Heal bot)
Routelocked is noting because both Mist and Rhys will go to same rout anyway, another 2 routes have Micaiah and Elincia, they don't need another healer.
-Better Magic Base
-Better magic growth
-Promotion isn't relient on a late Holy Crown. Not a Master Crown, but the 4-1 HOLY CROWN is needed for Mist.
All in all, Mists only advantage is additional move in p4, lolswords with a 25 str 3rd tier cap, and needed Florete/ALondite/Wyrmslayer/VK to do any kind of damage in endgame.
I guess she can Rescue Staff, but that's about it.
Also, mounts hinder themselves in Griel route, and mist being promoted by 4-4 means no ledge climbing.
And, RHys's support affinity gives out nice boosts too, being Fire.
And, it gives more offense and I think Hit.
now, what was that about being a better healer?