This tier is for COOP specifically, as honestly tiers do not matter in any other mode thus far--just use whoever you want (and that's what makes Rays so much more awesome).
Gonna be honest in that I don't remember a good selection of units, so there is room for a LOT OF CHANGE IN THIS LIST as time goes. If anything, characters are likely to just move up tiers as I learn and understand how to play them better, or as they get new artes to make them better. XD; This is the power of horizontal gacha!
Important things for Coop:
SPEED: Not only to make the grind faster in general, but the time limits for the 60 enemies (used to be 100!!) can be VERY tight depending on the enemy set + character set matchups.
(Tiermaker site is kind of ad-TRASH and bloaty, but is there a better place to get these? x_x) Also the watermark just ate Yggdrasil's face (who is supposed to be Mithos, but this list doesn't have them separated so)
Explanations for SS:
Mileena - Everything :3 Mostly Moon Serenade.
Shing/Kor - Shing's 空裂閃 Kuuretsusen at 275 (303) power is an actual instant delete button and I'm sad I don't see any other Shings use it.
Otherwise, he also has an aerial spam 5* 屠龍閃 Toryuusen at only 4 CC (with 6th enhance) to make him fly almost to the end of time... 8D
He will easily charge enough to be able to use his sgMA by the end.
Asbel - 葬刃 Soujin at 550 (825) power is an even bigger instant delete button, except it uses a much heavier 7 CC vs 4 and doesn't have IS break. BUT it also does explode into the Master Arte at 1430 (2145) power. So either way, OP.
Lloyd - He zips around with his R5. Aerial Shinkuuzan (why don't other characters have this enhance, I dunno) and easy to activate master 5*. Easy to get his sgMA ready. Just a powerhouse.
Mithos - Mostly for his default gMA lol. But honestly pretty good at instant deletions, and lots of powerful stuff overall.
Elize - She has a fast arte with GOOD RANGE in ティポサライブ Tipo Salive, using only 3 CC, which gives the absolutely broken 6th enhance "When chained to a spell, prevents staggering and negates damage less than 1000 during that combo". That keeps her safe to spam any of her massive spells. Both of her 5* spells are very powerful--I personally like Fearful Flare フィアフルフレア a bit more, but her original 5* リーベルイグニッション Revere Ignition looks AWESOME in its Master Arte form Bright Ignition ブライトイグニッション.
Rita - Same as Elize now with the recent 5*, AND she has Tidal Wave. 'Nuff said
Tear - Negate damage OP
Laphicet - Negate damage OP
Illia - Fast range master artes and has decent spell option (Spread) too... but mostly the range stuff. FAST RANGE...
Actually, there are a good number of S tiers that are SUPER close to qualifying for SS in my book (ex: Yuri, Estelle), I'd just like to get a better grip on them to see if I can consistently get their sgMAs for last enemy batch... Estelle would absolutely be instant SS if she had the negate damage enhance available.
Other units have it such as Raven, Malik, and Nonno G, but their setup to DPS is still slower compared to others from my experience... I just need more experience perhaps? XD;
-Dropped Ix from SS to S -> Shredding Talons invincibility is great for zipping around, but its DPS output suffers in the end and he doesn't really have better options for DPS in general.
Explanations for C:
Creed - His combo seems fine in practice, but in actuality, all of his spells come out as slow as dirt and don't have many hits or even damage to boast about. His base attack is also garbage slow. In order to give him more Iron Stance, one would need to waste a 3* slot (Indolence インドレンス) and even more time to set it up, not to mention that 3* arte injures himself in a mode where he doesn't really have a way of recovering that HP (except at checkpoints?). He's mostly serviceable if you use that 3*, but otherwise he's not a very fun character to play as if your partners are not great at occupying the other enemies. I definitely prefer a different kind of playstyle tbh. XD
Annie - Uhhh her 5* 戒めの一撃 Imashime no Ichigeki is literally her only melee arte which is DIRT SLOW and has an even SLOWER and less impactful Master Arte version, and all of her spells aren't really impactful either. I loved using Annie in ToR, but they haven't translated it very well into Rays. The biggest waste is that every arte besides the melee works into recovering her HP and those around her... but VERY FEW THINGS in Rays require that much HP recovery at all, and ESPECIALLY not in coop where huge HP recoveries happen at every checkpoint AND during benching. D: