The Fallen Dog God

Hyodo Issei is a (perverted) high school boy. When he goes out on a date with his first girlfriend Asano Yuma, he is killed by her. Her true identity is Reinare, a fallen angel sent by a secret society Grigori to eliminate dangerous divine weapons. Issei is reincarnated as a devil by his senpai Rias Gremory, a Duke of Hell. Issei serves as Rias's low rank devil and fights against the fallen angels.

Caught with a severe fever, Tobio Ikuse, a second-year high school student, was forced to miss his school trip on a luxury liner with his schoolmates. Four days later, Tobio hears the news about the disappearance of the teachers and second-year students from his school, and it devastates him. A few months after the incident, Tobio, having slowly recovered from the sudden loss of his friends, accidentally encounters one of them again. Chasing after his missing friend Sasaki, Tobio finally manages to catch upto him, but is shocked to see Sasaki alongside a Lizard Monster who attacks him together with the monster. Cornered by the monster, Tobio is rescued by another survivor of the missing luxury liner, Natsume Minagawa, who then hands him an egg before parting ways. Later that night, Tobio finds himself under attack, again, this time by two people with monsters. Amidst all the ensuing chaos, out of Tobio's shadow, came out a Puppy that rescues him. After fending off the two monsters, Natsume once again approaches Tobio before finally deciding to take him to their hideout.

Caught with a severe fever before his school trip, Tobio Ikuse, a second-year high school student was forced to miss out his school trip with his schoolmates on a luxury liner. Four days later, the news about the disappearance of the teachers and second-year students reaches Tobio's ears. Few months after the incident, Tobio having slowly recovered from the shock accidentally encounters one of his classmates again. (Source: High School DxD Wikia, edited)