Issei Hyoudou is obviously key. Then there are characters like Sirzechs Lucifer, the leader of the devils. Grayfia Lucifuge, Sirzechs' wife, also has an impact on the story. And characters like Irina Shidou, an angel and Issei's childhood friend, are important in the complex web of relationships and the overall plot of the High School DxD novels.
Well, in addition to Issei, there's Asia Argento. She's a nun - turned - devil and Issei's friend and love interest. Kiba Yuuto, a devil swordsman, is important. He has his own backstory and plays a significant role in battles. And Xenovia, a former exorcist turned devil, is also an important character.