Too many good 4*.

Keep reinhart especially since you have the best nature for it. If you get Leo or another good cavalry. Reinhart will be a very good character especially in Arena. As you already have Linde, i advice you to get a 5* dancer/singer. Olivia is insanely common and can be ugraded easily. She also have good speed and bulk. This can make her survive ORKO

inb4 upgrade Sharena - Your Nino and Reinhardt (especially Reinhardt) are good, I guess it depends on what are you lacking on the team.

You have probably the most optimal Reinhardt IVs he can have, so I'd definitely consider him. But since you have Linde and Clair, it would seem kind of pointless to upgrade him since you're already running two blues already.

It really depends on what you need right now, BUT, if I had the option to promote one of those characters, it would most definitely be Nino because of the fact that she isn't available as a 5*, while everyone else you listed is- sans Eliwood. Who is pretty decent I guess, but not the best mounted unit.

So yeah, Nino probably.

You have good options here. You have one of the best nature's for Tharja, although it's not common to see her in the arena too often. However, she does have her niche as The Murderer of Anything Green. Your Nino is also perfect for a Glass Cannon Build. I've never used Reinhardt myself, but it would seem that you have one of the best nature's for him too. But, to me, Cavalry are too easy to bait/eliminate in the Arena, unless you have a cavalry team. I generally stay away from Cavalry.

My advice as far as a team goes: Linde as a pseudo healer with Hone Speed and Rally Attack, Nino as a glass cannon, a dancer, preferably Olivia to help counter Hector as well as her Hone Attack, and try to find a tankier physical unit. Effie would be okay, but her movement doesn't match the rest of the team and she'll likely get left behind.