The Devil's High School Transfer Student

After traveling through time and reincarnation as a demon, he also gained a sister master, Rias, who claimed to be a high-ranking demon and often attacked at night;
Himejima Akeno, who is also a servant, always wants to reproduce with him;
In order to avoid physical strain and to stay alive, he worked hard to improve his strength.
Can you become stronger by signing a trade contract with humans? Then sign it!
Mai Sakurajima, the Nakano family's quintuplets, killer Yoel, Kato Megumi... all signed!
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 he is "invincible" in the world, or he is "invincible" in the world.