The Devil's High School Student

After traveling through time and reincarnation as a demon, he also has a master who claims to be a superior demon, Rias, who often attacks him at night;
Himejima Akeno, who is also a servant, always wants to reproduce with him;
In order to avoid physical exhaustion and to survive, he works hard to improve his strength.
Can you become stronger by signing a trade contract with humans? Then sign it!
Sakurajima Mai, the quintuplets of the Nakano family, the killer Yoel, Kato Megumi... all sign it!
In order to live a peaceful daily life with his harem, Long Yi, as a demon, said that he still needs to work hard, either to be "invincible" in the world, or to have no "enemy" in the world.