Akeno could also be a possibility. Her calm and sometimes mysterious nature might draw Kaido in. Their interactions could start with mutual respect due to their combat abilities and develop into something more romantic over time.
There's also the option of an original character being Kaido's love interest. This allows for more creative freedom in developing their relationship, without being restricted by the pre - existing character relationships in High School DxD. The writer can create a character specifically tailored to complement Kaido's personality and story arc.
Perhaps a mortal with a special gift for metallurgy or some other craft related to Hephaestus' work. This mortal could be seen as an equal in terms of skill, at least in a way that catches Hephaestus' eye. They would bond over their shared love for the craft, and over time, this bond could turn into a deep, loving relationship. Hephaestus might be drawn to the mortal's unique perspective and the way they approach the craft despite not having the same divine powers as he does.
Hinata is a very obvious choice. She has always had deep feelings for Naruto. In a fanfiction, their relationship could spiral further, exploring more of their shared interests, like their love for the village and their dreams for the future of the ninja world.
Issei is an obvious choice. Given his role as the main character in the series, he would likely be in the center of any story involving torture magic. Rias Gremory could also be involved as she often protects her comrades and would not stand by while they are subjected to torture magic. Akeno Himejima might be involved as well, especially if the torture magic has some connection to her powers or past.
If it's a crossover with the Percy Jackson universe, Percy himself could be interested in Ruby. They both have a sense of adventure and a desire to protect their friends. Annabeth might find herself drawn to Weiss because of their shared intelligence and strategic thinking. Leo Valdez could have a crush on Yang due to her mechanical skills and fiery personality. And Nico di Angelo might be intrigued by Blake because of her mysterious nature. These could all be great potential love interests in a RWBY - Percy Jackson crossover harem fanfiction.
Ezra might also be a possibility. He could be involved in a complex relationship with Spencer where he has to deal with her pregnancy and all the emotions that come with it.
There could be an older, more mature character, like a college student who she meets by chance and who is drawn to her unique charm and innocence.
It could be a figure skater. Maybe a rising star in the skating world who Bella meets at a competition and they strike up a connection. He might be drawn to her passion for the sport.
One possibility is Neville. He has always been a kind - hearted and brave character. In the fanfiction, he could be a knight or a nobleman who has a secret admiration for Hermione. As she becomes a princess, he might finally gather the courage to approach her. Or perhaps a wizard from a far - off land, who has heard of Hermione's intelligence and beauty and travels to court her. This wizard could bring new magic and different perspectives into Hermione's life, and a potential love story could unfold between them.
A plot could involve Kaido being sent on a special mission by the Devil King to infiltrate a human school. During this, he starts to develop real friendships with the characters in High School DxD. His initial motives are questioned as he begins to care more about his new friends than his mission, creating a lot of internal conflict.
Sakura could be a potential love interest. Since Naruto has always had a connection with her, as a girl, that connection could turn into a yuri - like romantic relationship. They already know each other well, and Naruto's new form could make Sakura see her in a different light.