High School DxD manga Series by Ichiei Ishibumi, Hiroji Mishima

When unpopular high schooler Issei Hyoudou has a less than romantic encounter with his first-ever girlfriend, the consequences are fatal! As he lies dying, who should come to his rescue but the prettiest girl in school, Rias Gremory! And the shocking truth she shares leaves Issei reeling... "You've been reborn a devil. Now you work for me!" Issei's uproarious life as a devil's minion begins here!

Summer's here, and before anyone knows it, the rating game between the Gremory and Sitri houses is scheduled to take place in the devil realm! Issei and the gang jump right into some hellish training, but something's not right with Koneko-chan...?