The light novel, manga, and anime series High School DxD features a diverse cast of characters. The visuals of the characters were designed by Miyama-Zero and their stories were created by Ichiei Ishibumi. The stories follow the adventures of Issei Hyodo, a perverted high school guy who is killed by his first date, but reborn as a devil to serve Rias Gremory, a crimson-haired school beauty who heads the Occult Research Club (オカルト研究部 Okaruto Kenkyū-bu?). Issei, Rias, and the club members interact with various groups and organizations, some of which are run by devils who compete against them in sanctioned combat matches called Rating Games, where the characters have been assigned positions akin to chess pieces.
Author Ichiei Ishibumi had previously worked on a horror genre series two years before publishing the first light novel for High School DxD. In the volume 1 afterword, he mentions that he changed his writing style, and wanted to develop one in the school-life, love-comedy, battle, and fantasy genre. His main character Issei was made into a lecherous guy at the request of his editor. For the first volume, he made Rias a second lead character and heroine, and Asia to be the second heroine. Some of his characters are loosely referenced from the Bible, non-fiction books, and mythology. He created a world where the war between the three main factions (angels, devils, and fallen angels) is over, and where many famous angels and devils have already ceased to exist. He also added characters and legendary beasts from a variety of places from Norse mythology to Japanese yokai. Ishibumi named the Four Great Devils after characters from the Gundam media franchise. For example, Sir Zechs Lucifer is a combination of Char Aznable and Zechs Merquise, and Millicas is named after Merquise and Aznable's real names Milliardo and Casval.