Maou-sama no Konwaku

The protagonist is a devil king, and yet there's nothing repellant about him.
He's smart, capable (one might say showy) and not at all gruff or evil toward anyone.
He keeps a tidy castle and enjoys playing baseball and things.

He also has a personal servant named Milly-chan.
Milly's been a devoted minion-maid for generations of devil kings.
She's begun to slip into darkness lately... the erotic kind.

[From DLsite English]