Dancer

Well, every dancer is helpful. All have their own niches
HS!Ryoma is semi offensive, being able to take Armors with ease. Even a color advantage is some situations.
Performing Olivia can take use of almost any dagger perfectly. And can run some interesting builds depending on what you use.
Silvia can be an absolutely amazing res tank and can take a hit or two if you need her to as she's fairly fast and can avoid doubles. Just don't expect her to do much in combat.
And all of them of course have their own dance buff like Blaze Dance and so. So all of them can be useful in certain, if not, most situations.

Okay, Ryoma is more offensive. Semi offensive depending on IV's. He can be a great anti red if your team needs one and his speed normally lets him do that (also armors die hard). I'd recommend building him with Death Blow if possible, and making it so he can one shot armors that are blue/red/colorless with not much effort on the side, while his main job is dancing.

Siliva is a ploy beast. If you can, I'd recommend giving her Atk Ploy on her C Slot, and Def Ploy on her S. Fury on her main, keep her weapon and refine it for res. Wings of Mercy and Iceberg. With this setup she can ploy half the world, and she can take one some dragons if you really need her to, which may happen given how strong they are.

Performing Olivia is a great unit, and there's a few things you could do. You could make her anti infantry (Death Blow with Poison Dagger+ if she's +atk), a smoke dagger to debuff everyone around her, and making your teammates lives easier. Or running the cheap Anti Mage build. Keep her base dagger (preferably a Barb shuriken), res refine it (or Spd if you wanna outspeed them, which may be better), Keep her A Skill DD3 (or give her Fury/Mirro Stance), Wings of Mercy for general use and maybe something more specialized later. For her S Slot I could recommend either Deflect Magic (she survives certain Rein's with this), or Distant Def 3 which maximizes her combat against other mages. Also, either Iceberg (if you go +res on that refine) or moonbow to chip through higher defense enemies, since she doesn't have wonderful atk.

Silvia is amazing let me tell you. 45 health (after refine) and 30 base res (34 after refine. 37 if boon). She can run double ploy, double drives, or double hones. She's a good mage bait. A slot would be Fury 3 to get her speed and res up of course. Damage potential isn't too great though