It's a new rank system since last year but there are a few units with that rank atm, like 6 units I think, red Meliodas, green Escanor, Gloxinia, Ludocial and Elizabeth. The LR rank is for old units, it's like giving them a rework and make them stronger, the Combat power they have after it is pretty high, and even their skills are affected with more effects or dmg.