Harem fanfiction is a form of fan - made literature. In it, there's a central character, usually male, around whom several other characters, mostly female, revolve in a romantic sense. It draws inspiration from existing fictional works. For example, if there's a popular anime with a male lead, a harem fanfiction might be written where that lead has multiple female characters interested in him, adding new storylines and relationship developments that may not be in the original work.
Rukia could be in the harem. She has a connection with Ichigo already and her strong personality might be drawn to the power and mystery of Hollow Ichigo. Orihime is also a likely candidate. Her kind and loving nature could be what makes her want to reach out to Hollow Ichigo and be part of his harem.
In such a fanfiction, Princess Bubblegum from Adventure Time could be in the harem. She's intelligent and powerful, and Naruto's unique skills and personality might attract her. Also, Sakura from Naruto could be part of it. Since she has a history with Naruto and in this crossover, she could be part of the group vying for his attention. And Marceline too. Her cool and edgy nature might be drawn to Naruto's heroic side.
Potential members could include Tenten from Naruto's world. She might be interested in Naruto's adventures in a different universe and become part of the harem. In Starfire's world, there could be a scientist - type character who is fascinated by Naruto's ninja abilities and the concept of chakra. This character could start to develop feelings for Naruto as she studies him more closely, thus joining the harem.
Sakura could be a potential member. She has a long - standing relationship with Naruto and her skills as a medical ninja could be an asset to the group. Ino might also be part of it. Her mind - control jutsu and her strong personality could add an interesting dynamic. Tenten could be there too, with her weapon expertise. These characters already have connections to Naruto in the original story, and in a harem fanfiction, they could form a unique relationship with both Naruto and Kurenai.
Definitely Hinata. Her love for Naruto is well - known. Sakura, too, as she has grown closer to Naruto over the years. Ino might also be in the harem. She has had her own moments with Naruto and her skills as a ninja could be an asset. And perhaps Karui from the Cloud Village. After Naruto's peace - making efforts between the villages, she could have developed feelings for him.
Well, obviously Rias herself would be a central member. Then there could be Akeno Himejima. She has a mysterious and alluring personality, and her relationship with Rias could be an interesting dynamic when Naruto enters the picture. Also, Koneko Toujou, with her cute and somewhat tsundere nature, might be another member. She could be initially cold to Naruto but gradually warm up to him.
There could be some original Arrancar characters created specifically for the fanfiction. For example, a young and curious Arrancar who is new to the ways of her kind. She meets Naruto and is immediately fascinated by his different appearance and abilities. And then there's a more experienced Arrancar who has seen many battles. She initially sees Naruto as a threat but then realizes his good intentions and slowly falls for him, becoming part of the harem.
Master Chief is often a central figure around which a harem might form. His stoic and heroic nature makes him an attractive character for this kind of fanfiction. Cortana could also be part of the harem, as she has a deep connection with Master Chief. And Arbiter might be included in some cases as he represents an important alien species in the Halo universe and his character has depth and complexity.
Well, Mumen Rider could potentially be in a comical sense in a One Punch Man harem fanfiction. Just kidding! But seriously, characters like Fubuki are prime candidates. She has a strong personality and her own motives which could easily be intertwined with romantic feelings for Saitama. Also, any new female characters introduced to the One Punch Man universe in the fanfiction could be part of the harem. They might be normal civilians who witness Saitama's heroics up close and become smitten.