Berkut Horse Emblem

Green is a little harder because they all need a heavy amount of investment. If you don't already have Reinhardt, I recommend you use Cecilia with Gronnblade from a Nino so you have a source of magic damage. Also, I recommend you don't use Berkut. I tried to use him yesterday and it just doesn't work out so well. He's a cool character and you can just use him for fun I guess, but I wouldn't invest heavily in him. Even Clive might be better when he comes out

You should have a Cecilia laying around from the HB unless you used her for inheritance. She's not a stellar unit but she can be useful in horse emblem. I run triangle adept/raven tome/bowbreaker on mine. Granted - Ursula is my nuke with an inherited blade tome so that let's Cecilia play more of a check/support role.

Cecilia is the only green mage, and a halfway decent one at that, so she's your best bet. As stated above, Berkut isn't great--you're better off using basically any other mounted lance, honestly. If Camus gets a revival GHB, he's The Best, but Roderick and Abel are also good / relatively common. If you don't want to use Cecilia, Frederick is another strong option. Dude's STRONG.

Everyone says to use Cecilia, but I think Frederick is also very much a fine option. I don't even have a horse emblem team for him yet but he still does great with a regular brave axe. Of course, it depends if you need magic damage or physical damage.

My current horse emblem team is Xander, Camus, Ursula, and Frederick. I tend to focus on physical damage, so if that's your thing, I totally recommend Frederick for your green slot.
My Frederick is +ATK/-HP so I gave him a Brave Axe+. I have and extra Klein, so if I don't end up using Deathblow on someone else, I'll give it to Frederick.
I'm pretty disappointed in Berkut's stats. I was really hoping I could switch out Ursula, but I'll just end up keeping her. You could take Berkut out and have 1 red, 1 blue and 2 greens since there are a lot of blue cavalry units.

Frederick and Titania are decent green options for a horse team.

Frederick requires heavy investment with good IVs (+Atk - Somthing), with a braxe axe and all. But he is really good.

Titania is easier to use. Just max her and kill blues, resisting some non red mages.

Cecilia is a good magical option, if you have what it takes to
* build her (NINOS)
* buff her (hone + fortify cavalry)
She is soooo slow, but one shots almost anything with 55+ dmg

You could also use lancebreaker on Berkut and put a blue blade tome Ursula/Olwen so you don't have to invest on a green unit. I'm trying to fit him in a Horse Emblem team too, along with Xander, Olwen/Ursula and don't know who will take the last spot. Maybe a healer, as Xander and Berkut can tank pretty well and the blade tome user can nuke.