Celica boasts almost identical stats to Tharja, only giving up 1 Spd and 1 Def for 2 more Res. With a Raudrblade tome she should thus perform almost as well. However, you also have the option of running her with her personal new 14 might tome - Ragnarok, which boosts her into overdrive with 37 Atk and 38 Spd as long as she is at full health. If you pair this with Fury 3, she has 40 Atk (54 with the tome) and 41 Spd. After initiating combat once she loses 11 HP, which is enough to get her into Desperation range. If you have pulled a non -Atk or -Spd Celica I would fully recommend trying out this build. Her ceiling is lower than that of any blade mage, but she doesn't need buffs to be lethal.
However, she is still red which is not the best color for fragile, offensive magic and her enemy phase is atrocious. Thus, she shouldn't at all be considered for S. This is a very clear A.
Before seeing her stats I expected Mae to be a glass cannon blue mage in a similar vein as Linde or Lucina. However, with her 31 Spd, 30 Res and 36 Atk she actually fills the until now empty niche of blue resistance tank, which makes her comparable to the likes of Sanaki and Julia. However, she lacks a powerful personal tome and, considering the current blue and green magic filled metagame, finds herself in a questionable position. It's possible that she will see more use once red magic trends upwards, but right now there is little reason to put her a whole tier above Sanaki. I would recommend Nowi over her for a very similar role. Thus, Mae belongs in B.
Boey is an attempt to counteract the offensive 1RKO arena metagame by introducing more physically defensive mage options. Boey makes okay use of the tome he starts with, and Gronnraven would be another solid option. However, he compares unfavorably to female Robin for the same role, boasting +3 HP/-2 Spd/+3 Def/-4 Res. Losing 4 Res and 2 Spd in favor of HP and some physical bulk is not at all justified in my view, and there is absolutely no reason to put him a tier above Robin. Thus, Boey undoubtedly belongs in C.