My current prediction is that Reinhard is the ultimate badguy (or working directly under the ultimate badguy).

There are a few reasons for this:
The ultimate villain should be introduced early, and not just be some random new character.
He's way too nice and way too strong.
He's supporting Felt, who wants to overthrow the kingdom, for somewhat mysterious reasons.
Supposedly he's as good as this Swordmaster guy, and the Swordmaster supposedly died to the whale that erases your existence when you die to it. Yet, we know about the Swordmaster. Reinhard probably is him.
It would be interesting and also present another good tragic/suffering moment for Subaru.