Write an interesting, riveting and lengthy short story in the form of a crossover between Pokemon: The Series starting from Black and White, Bakemonogatari, and High School DxD, in which Ash Ketchum/Satoshi is actually a young Issei Hyoudou. After becoming the Pokemon World Champion. Ash/Satoshi decided to retire from Pokemon training and return to Pallet Town in Kanto to try and live life as a normal elementary school boy under his real name, Issei Hyoudou. The Hyoudous then moved back to Kuoh, with nobody except Issei/Ash/Satoshi's mom Delilah/Miki being aware of Issei's true past as a legendary Pokemon trainer. Issei still continues to give lots of love and care to his Pokemon and helps them keep in shape by battling the various low-level yokai, stray devils and rouge magicians around Kuoh as he goes through elementary, middle and later on high school at Kuoh, he just doesn't do professional Pokemon training anymore. Serena is Issei/Ash/Satoshi's younger half-sister and replaces Sengoku Nadeko and has an incestuous crush on him that she tries to keep hidden. 
Tsubasa Hanekawa replaces Sona Sitri, and also has Sona's positions as Student Council President, head of the Sitri peerage and heiress of the House of Sitri. Even after his reincarnation by his English teacher, the seductive Himekami Lizel, as the pawn of Lizel's true identity, the heiress of the House of Gremory and defacto Princess of the Underworld Rias Gremory, Issei manages to keep the fact that he was once a Pokemon trainer as a young boy hidden from both his fellow members of the Gremory peerage and the rest of the Kuoh student and teacher body. This ends up backfiring on him when Lillie and Mallow manage to track him down to Kuoh with the help of Professor Oak and pay him a visit during the Kuoh art exhibition, in order to show their gratitude for helping them out during the Hyoudou's family vacation in Aloha [Hawaii] when Issei was just a fifth-grade elementary school student.
 Issei is barely able to hide the truth from the students and teachers of Kuoh but Hanekawa and Himekami/Rias are able to figure out what really happened via deduction and intelligent guesses and confront Issei about this. Issei reluctantly tells Rias/Himekami, Rias' peerage and Hanekawa the truth about his childhood during an impromptu meeting of the ORC, and Rias/Lizel and Hanekawa forgive Issei for hiding the truth from them under the condition that he has to marry all the girls in his harem and get them pregnant, starting with the Sitri and Gremory heiresses. This rather bold declaration causes Lillie and Mallow, who are also present at the meeting, to blush heavily and have erotic daydreams about Issei dominating them in the bedroom. 
Apart from Himekami/Rias. Hanekawa, the female members of both Himekami/Rias' and Hanekawa's peerages, Rossweisse, Kuroka, Kunou, Ravel, Le Fay, Riser's peerage, Irina, Xenovia and Ophis, Issei also has Miette, Serena, May, Misty, Iris, Bea, Jessie from Team Rocket, Lillie and Mallow in his harem after Rias/Himekami and Hanekawa use their familial wealth, position and connections to contact the Pokegirls and tell them about Issei/Ash/Satoshi's location, in exchange for joining his harem. Hanekawa's older sister, the famous female college basketball star Suruga Kanbaru, also joins Issei/Ash/Satoshi's harem after Rias reveals that she [Kanbaru] is actually Serafell Levithan, one of the four great kings of the Underworld. Issei/Ash/Satoshi's vampire familiar Shinobu Oshino, better known to the magical world and the higher up Pokemon trainers as Kiss-Shot-Under Blade, the Witch of Babylon, also becomes a part of the harem as well.
The story ends with Vali and the Khaos Brigade deciding to pay Kuoh a visit in order to cause chaos and make their name known throughout the world, only to be beaten almost to death and run out of town by the enraged girls of Issei's vast harem. From the safety of behind a corner, Red, the protagonist of the original Pokemon games, looks at the whole thing with amusement before commenting that Ash will most likely father a lot of children rather soon, before leaving to deal with business, unseen by either Khaos Brigade or Issei and his harem.