Altair is the best ssr in the game, but is a dude, Birdman really only works with certain classes, Sarunan is ok but also a dude, which leaves you only with Le Fay, who is unfortunately weak, and Lily, which is actually a great choice and can help you beat the fire hell pinya easily.
Depends on what do you want for your team. If you need healer, go for Lily. Her heal is one of the better heal since it also removes 1 debuff. Her fire damage cut skill is nice for fire boss as well, though it has really long CD of 10 turns.
If you need debuffer (so you won't get stuck with Miserable Mist EX/Dark Fencer) go for Altair. His 3rd skill is great too since it gives 30% ougi gauge to everyone.
Socie is also a great choice since she has both 'Miserable Mist' like skill (like Altair) and Slow. Her heal is also nice but it's only single target so you might run into trouble with prolong boss fight with heavy hitting AoE/multiple hit normal attack boss.
Personally, I think you should pick Altair since Catalina can act as pseudo healer/tanker (her heal is moderately good and she has damage cut skill plus barrier skill) and his Ougi boosting skill is too good to pass up. But if it's me, I will pick Socie since I like her character design... efficiency be damned!
Ah, you mean other SSR you can pick are those? Then go for Altair. He's the best among them.
Birdman is only good during break state skill + ougi to boost damage. (though he hit really hard when conditions are met) Le Fay is ok since she hits damn hard for a healer, although her regen skill has too long CD which defeat the whole purpose of having healers. Sarunan is quite ok with his break lock/dmg increase and his first skill gives 30% dark dmg reduction for 3 whopping turns. Only bad point about him is his antic with his summon...
I second/third Altair. He got a really awesome voice too (OnoD's), and his buffs are great + stackable on most parts.
Lily is really cute imo, but has one of the lowest overall stats at Level 80 for an SSR. Her barrier only works against fire-type enemies, so she is most often limited to being stuck in a water team - which shouldn't be a big problem, unless you are expecting versatility and flexibility.
Sarunan's second skill (lock on break) is cool, but for the most part you won't get to use it unless you occasionally find yourself solo-ing bosses with a mode gauge. In raids, you can just rely on someone else to extend break for you hahaha.