I have all the characters maxed with their go to class and inheritance and my Celine Vidame&Chrom is hand down among the best damage dealer in both mag and str damage when skill proc ( more often than not ), with the 2 range magical sword she deal around 90*2 per turn with self buff and the physical 1 range critical sword can reach 300 in crit if advantage + proc. if you prefere Range ==>Byleth, Damage ==>Chrom Use +5STR/5DEX - +5MAG/+5DEX to maximize the proc of her class skill

Celine has a special Synergy with Sigurd. Celine only lacks stats. She has low defenses, dex, build, and mobility. Sigurd directly fixes these issues. Celine has a unique aspect to her, she is the only [Mystic] class unit capable of using Swords. Meaning she has unique bonuses to some emblem rings. For Sigurds Engage Overdrive, can only be used by Swords/Lance's, and the [Mystic] bonus to this, adds [25% Magic damage]. So Celine uniquely can use this boosted Overdrive. Levin Sword is good too.

Obiously the best pairing with Celine is Byleth, and the best for Byleth is Celine. When engaged she easily surpass Hortensia as a healer and also become a powerfull magic unit with +2 range or a decent (at the very least) melee unit with Byleth sword taking into account her magic and physical points. With all that and considering her personal skill and her class skill she should be at least A tier.

I agree that sword makes her a tier but let us all agree she should be bow over healing

I have been using Céline with Byleth and I think they fit well together. I slap on levin sword and wind on her with sword agility and break defenses, which can be useful with the vajra mushti to break mages, archers and thieves. She’s an evade tank and counter for mages.