To go into more detail (forgive the meme spacing, I want this wall of text to be easily readable)
Altair is probably the best overall buffer in the game. He does everything, he has a miserable mist that also lowers the enemy water defense as well (at level 90), an attack and defense buff for your whole team and water attack up as well (at level 100), and his third skill is a straight-up 30% boost to everyone's charge bars. Him being on your team also boosts everyone's debuff success rate by 5%. Combine that with his ougi giving your party a 70% power 20%-proc critical chance, and you have a unit who isn't going anywhere any time soon. I have both 5* eternals for water, and he's right on the front lines with them in every single fire raid I do.
Siegfried goes from zero to hero from level 99 to level 100. He's a glass cannon pre-100 and functions as more of a break assassin. His first skill is a basic nuke that does decent damage and gives him 20% water cut. Second skill is a 50% attack buff and 50% defense down debuff on himself. His third gives him 25% charge and cuts the enemy's mode bar. However, at 100, this all changes. His ougi starts giving him heavy buffs, and it'll echo outside of break. His 1 gives all his allies 20% water cut instead of just himself, his 3 gives his 20% DA up as well, and his 2 not only changes the attack buff to 60% and makes the multiplier unique (even more damage), but it reverses the defense down on himself into 30% defense up on himself. He essentially goes from glass cannon to a durable, hard hitting tank. Did I mention his passive gives him a chance to get some HP back on himself when autoattacking? He's durable as fuck, has a sword proficiency, and doesn't afraid of anything.
Percival suffers less from being subpar pre-100, but his 100 still spruces him up a fuck ton. His 1 is a small nuke that at level 90 heals him for 800 HP. His 3 gives him a Strength buff (more damage when you're at high HP) and at level 100 gives him 30% fire attack up and 20% echoes, which enables his passive, which gives him an extra 30% fire attack buff when he has a fire attack buff. His 2 if a show-stopper; it inflicts a 60-second Fear debuff for everyone in the raid, in which the boss can't gain charge diamonds or use ougis (triggers will still activate though.) His other passive is basically a free 15% attack, 10% DA and 6% TA when theres 3 other allies still alive with him. In addition, his support EMP is the chance the burn an enemy with autoattacks, which is a beautful synergy with a xeno ifrit axe mainhand. His damage and unique supporter ability makes him a wonderful addition to any fire team.
Vira is a cutie and she'll treat you well. Pre-5*, she's a veilbot with her 2, plain and simple. Veil is a precious commodity in dark, so her having it already makes her extremely relevant in any raid lasting more than a few turns. At level 100 this gives you two turns of 100% TA, so its also very nice for that purpose. Her 1 is a baby miserable mist that inflicted 15% att and def down, and also buffs herself for 15% attack and defense. Ain't much to say about that skill. Her 3 is an attack skill that does a decent amount of dark and light damage, and heals her for 2k at level 95, 1.5k before. She gets really nutty at level 90 though, as she gains a 4th skill in Luminiera Merge, which not only gives her light echoes to all her autoattacks, but gives her a 10% uplift, 15% more def and attack, 50% DA and 15% TA, and she takes a bit less damage from dark enemies. She's a veil user with a fair offensive presence, but her healing skill andsupport EMP which lets her recover HP sometimes upon autoattacking makes her have a little bit of anti-synergy with enmity and zoi. However, iif you don't take EMPs, you can use her in enmity lineups a bit better.
tl:dr altair a best buff, siegfried a tank, perci a damage fearbot, vira a veilbot