Boros players will be happy to play Rem Karolus in any of their aggro shells. It's got a decent body and mana cost, and all effects are great.
Damage prevention is an especially big deal, as well as, adding that one extra damage to your own spells. Playing this against a Mono-Red Aggro would be a sight to behold, when all their spells get nullified.
It can be compared to Mantis Rider, a staple in modern humans, but this one also has some serious upside to it.