I love analyzing turn based combat, basically since I got into Persona, then SMT and other games. I'm usually not amazing at detecting what's broken but so much is strong and easily accessible in this game that I feel I can make decent judgements. I do think that this likely changes a few of my opinions; I'm going to take a very strong look at utility and extra usefulness since at least with my playstyle, I very much look to abuse it.

S++++: Tressa Donate BP is... hilariously strong. Even from level 3 or whatever the normal point to get the JP to buy it, it makes her essentially the most consistently strong support in the game since she essentially becomes a power amp for like... 3 SP. She doesn't need to do damage or heal or attack or do basically anything else to be useful, she just outright makes everyone else in the party stronger as needed with almost no downside. Hired Help is incredibly high damage at almost all points in the game although it is expensive to use. Sidestep and Rest are great utility as well and she doesn't even need it. And to top it all off, she basically passively generates money for you as you play with merchant passives. The extra + in my tweets come from finding out yesterday that there's apparently an ability that allows her to apply her self targeted abilities on the entire party, which makes her the best healer in the game with rest and nullifies half the enemies in the game with side step. Basically, she has almost no flaws. I see no other combination of three characters that would not be improved more with Tressa than with any other character.
S: Ophelia - AoE healing, although at a slightly high SP cost. I would struggle to see a stronger ability than her ultimate which almost singlehandedly puts her at this spot, combined with healing utility (she can make someone else's actions go twice for the next 3 turns.
S-: Primrose - Generic buffs but they're all very good. Buffs are very strong. At least in my files, she does very good elemental damage as well. Dancer passives, especially the regen and extra duration on buffs are among the best I've seen.
Therion - Debuffs, and low damage but multihit specials (multihit usually leads to more potential damage). Purple chests are nice but being able to completely loot a town by stealing everything that's not bolted down creates economy in game breaking ways.
A+: Cyrus - Double hit elemental damage nukes. Not much else here really, although he does get outclassed by a hidden class. The avoiding encounter passive is super nice for moving around though.
A: H’aanit - Early game goddess, thanks to being able to summon Linde for free. She basically two shots every encounter until chapter 2. With time investment, capturing monsters is really solid. Leg Hold trap is a deceptively good support ability as well, and the multi hit arrow attacks are amazing against anything with an arrow weakness, exposing them for your primary damage dealer. Basically, she starts as a damage dealer but transitions into a very serviceable support in mid game.
A-: Alfyn - This is the one I'm least confident in. He could be anywhere from here to S. He can be a better Tressa if he's well supplied but that costs money and resources and I'm not a fan of leaning on pomengranate essences which are not purchaseable. He's a character that very likely is among the strongest in a max level, max everything party, but I'm worried about the resource  commitment and how he could potentially struggle to contribute elsewhere.
B+: Olberic. - He's okay. Doesn't have that much utility outside of being able to poke a lot of weapon types and a similar multi hit ability to H'annit. He does hit very hard so if you were wanting to focus on physical damage, you could do worse than to pair him with Tressa, Ophelia and Therion. I just think he's a bit more limited in toolset than the others and you want pretty specific things if he were to be a major part of the game.