Help building Horse emblem

Reinhardt, B.Roy, Elise, Frederick maybe...Frederick will be your tank with QR (brave axe is still good), and B.Roy has galeforce which provides another dimension of offense to the team, and Elise can heal him up if he retreats after activating galeforce (otherwise, he won't be able to activate galeforce in one turn anymore).

When I play horse emblem I really prefer ranged units because 3-tile movement + 2 range covers the width of most maps. Since you don't have strong mages or B.Lyn, we turn to other options.

You have really good options on red. Xander is particularly attractive, since he has respectable bulk and a DC weapon. Siegbert would need a new C skill to invest in (since a tactic is nigh useless in an Emblem team). Ares is a particularly top tier choice. Been running him for a while with Vantage and he can clear maps all by his own. And Brave Roy is exceptionally powerful with stuff like Life and Death, but he'll be *very* fragile. Can't really go wrong with any of these. I've ran them all.
Xander: Great physical tank and coverage. Slow
Ares: High damage potential in longer battles (he's a beast once he gets his first special proc).
Roy is more of a sweeper. Get in, get out. He won't stand much during enemy phase.
Siegbert is a mixed bag for me, but he's by no means unreliable. He's overall very rounded.

When it comes to blues, I'd say it's a pick between Summer Cordelia and Camus (since you've already decided on Rein). Camus is very much like Xander, except he trades in some physical bulk for better speed, which makes him very good in both phases.
Summer Cordelia, however, is another sweeper. Her speed is her most valuable stat and you can reliably let got of her defenses, which I recommend with the IV you got her with. However, she'd need a complete overhaul. A skill like Swift Sparrow or LnD would suit her way, and a preferable Desperation for when she does get hit.

Marth, I'm guessing refers to Groom Marth. In which case, I'd recommend him over all other greens you have, except for one in specific.
His defense is solid, but not exactly the best. His best role is as a support. Ardent Service, however, would need to be replaced in a cav team, and so would his drive. Slaying Axe is a good rec.
The other green cav I can recommend is Titania, as she has a particularly high res and you don't seem to have many res tanks. With her Draconic Poleaxe, she can easily tank Reinhardt. Don't go for the specialized refine though. Again, tactics are useless to you rn. I'd say give her an attack refine to patch up her not-so-great attack stat.

What I'd choose:
Red: Xander or Ares/Roy.
Blue: Camus (if you didn't pick Xander) or Summer Cordelia (if you did pick Xander).
Green: Titania (again, res tanking is something you lack) or groom Marth (if your playstyle is more aggressive and need less baiting)
And, of course, Reinhardt, as you already mentioned.

EDIT: Sorry for the wall of text. I tend to talk too much.

Xander Tank I recomend Ignis +G Tome Breaker
Titania Res Tank + blue check I recomend prf weapon, fury, glacies, hone cav.
Reinhardt Nuker Standart Build
Elise Healer. Razzle Dazzle + Double Savage blow

Thats the team i will use.
Covers the Color Spectrum and works good together.