Main Character:
Takayama Kaito / Lucifer
Heroines:
Koneko / Shirone (Highschool DXD)
Shiina Mashiro (Sakurasou no Pet na Kanojo)
Hayasaka Ai (Love is War)
Relevant Secondary Characters:
(These may become future heroines, recurring characters, or important figures within the development of the lore.)
Shinomiya Kaguya (Love is War): A lord subordinate to the protagonist, responsible for handling politics, administration, and similar matters, as well as teaching those to the protagonist.
Lala: A lord subordinate to the protagonist, responsible for inventions. Her father also seems to have some kind of business or matter with the protagonist.
Kirigaya Kazuto / Kirito (Sword Art Online): A lord subordinate to the protagonist, possesses an ability that allows him to create sword techniques, almost at the level of real skills, which others can also learn.
Sorata Kanda: Angry at the protagonist for "stealing" Mashiro, although he has already lost his life to him twice. He will probably return in the future as an enemy of the protagonist.
I want to clarify that I will not simply turn all the original protagonists into villains just so the main character can end up with their heroines. There are different cases. For example, there is Kirito, who is with Asuna and the protagonist does not interfere. Then there is Tomoya, where the protagonist does not come between him and Eriri, but he is with Kato. And then there are cases like this one, where Sorata becomes a villain. Though this might just be my opinion, I think it makes sense within the canon. In the series, we saw how Sorata fell into darkness and acted aggressively due to his inferiority complex in the dorm. Mashiro was the one who pulled him out of it. Now imagine Mashiro leaving as well, causing him to fall again, and this time he has powers.
Arata: Former second place on Japan's Server 26 (the same as the protagonist). He faced the protagonist during the Battle of Lords, lost, and is now in a mental state that prevents him from improving. Has basic temporary powers (within that category). He will return and be very important to the story because I love everything related to time.
Asuna: A lord subordinate to the protagonist, in a relationship with Kirito, more or less.
Tomoya Aki: Only appeared briefly in the Battle of Lords, so he does not have any connection with the protagonist yet. Has powers similar to a Green Lantern's but powered by mana.
Eriri Spencer Sawamura: Only appeared briefly in the Battle of Lords, so she does not have any connection with the protagonist yet. Has general magic abilities and is in love with Tomoya.
Makima (Chainsaw Man): Based on the character of the same name in terms of appearance and personality, although the story of Chainsaw Man does not exist in this fic. She can be considered this world's version. She is the receptionist of the guild (in the protagonist's city branch) and a rank 4 demon.
Sonya (Original): A dark elf, former member of the royal guard of the elven kingdom in the Dark Forest. Level 4 mage in shadow arts and mentor to the protagonist in this discipline.
Sylver(Original): A two-headed Cerberus, leader of the Hellhounds under the protagonist's command. At first, he was accompanied by his pack of silver wolves, but they evolved thanks to the protagonist. He also gained the ability to summon more, so his pack continues to expand constantly.
Lorim(Original): De facto leader of the beastmen under the protagonist's orders. Originally the chief of a kobold village subjugated by Kaito. After joining, they received a tower where, by overcoming trials, they can evolve into beastmen. Although the population has grown exponentially and Lorim is no longer the chief of the original village, most still recognize him as a guide and trusted figure.
Korran(Original): Captain of the beastmen combat force. He was the only kobold who dared to challenge the protagonist upon his arrival, thus earning his recognition. He currently leads the warriors of his race, although not all kobolds and beastmen choose that path.
Ben(Original): First human summoned by the protagonist and later transformed into a demon. He is the de facto leader of the demons under his command. Although each of the 72 demon houses has its own leader, Ben serves as the main representative before Kaito.
Nyssara(Original): Lady of the city of Nareth, the first elven city conquered by the protagonist. Expert in wind spells. Younger sister of Sonya.
Ryselle(Original): Lady of the city of Veldris, the second elven city under the protagonist's domain. Specialist in fire spells. Younger sister of Sonya.
Colette(Original): Lady of the city of Velanyr, the third elven city conquered by the protagonist.
Viktor Asano(Original): Leader of the guild branch in the protagonist's city.
Hikaru Saito(Original): The protagonist's best male friend in the real world.
Aoyama Nanami (Sakurasou no Pet na Kanojo): Close friend of the protagonist.
Takayama Sora(Original): The protagonist's younger sister.
Takayama Sayuri(Original): The protagonist's mother.
Frey(Original): Under the protagonist's command. Member of the Phoenix house, occupation: tailor.
Kamiigusa Misaki: Mashiro's friend and dormmate. No connection with the protagonist beyond that. She feels somewhat guilty for accidentally causing the conflict between Sorata and Mashiro.
These are the most relevant characters so far. Some lack additional information because it has not yet been revealed in the story; this list will be updated as needed.