Renji could also work well. He's always been in the thick of things in the Soul Society. His rough - around - the - edges personality could contrast with the more subtle relationship between Ichigo and Lisa. He might be the one who accidentally stumbles upon their budding relationship and gives some comical reactions.
Rock Lee would be an exciting supporting character. His energetic and determined personality could bring some humor and motivation to the story. He could challenge Naruto to friendly sparring matches, which in turn could make Ino and Sakura cheer for them. Choji is also a good option. His loyalty to Ino and his friendship with Naruto could create some heartwarming moments. And Tsunade could play a role as well. As the Hokage, she could assign them important missions and also be a mentor figure, especially to Sakura, which would impact the relationship between Naruto, Ino, and Sakura.
Kakashi would be an interesting addition. He could act as Naruto's advisor in matters of the heart. Since Kakashi has a good understanding of Naruto's character, he could give Naruto some valuable insights on how to handle his relationship with Shizune. At the same time, Kakashi's calm and collected nature could contrast with Naruto's more impulsive self, making for some engaging interactions within the fanfic.
Polly could be a great supporting role. Since she has a history with Piper, she could bring in some family - related drama. Also, Red would be an amazing addition. Her motherly nature towards Piper in prison could translate into her being a sort of mentor or a source of advice in their married life.
Haley would be a great supporting role. She could offer Peyton advice on pregnancy based on her own experiences. Lucas could also be included, perhaps he helps Nathan with getting ready for the baby, like fixing up a room in their house. And Brooke, she might be the one to throw a baby shower for Peyton and bring in all the fun and glamour to the story.
Riko could be a great supporting character. As the coach, she can play a role in bringing Hyuuga and Kuroko closer. For example, she might assign them special tasks together which force them to interact more. Also, Kagami could be included. His competitive nature and his friendship with Kuroko can add an interesting dynamic. He might be initially jealous of Hyuuga's relationship with Kuroko, but then come to understand and support it.
Annabeth would be a great supporting character. She could be initially suspicious of Miss Martian, thinking she might be a threat to Percy. This would create some tension and drama in the story. Grover could also be included. His kind - hearted nature would make him open to befriending Miss Martian quickly, and he could act as a mediator between Percy and Annabeth when it comes to their relationship with Miss Martian. On Miss Martian's side, Martian Manhunter could play an important role. He could be the one who gives Miss Martian advice about her relationship with an Earthling, and also help her understand Percy's world better.
Definitely include Natsu. His enthusiasm and positive energy can add some light - hearted moments to the story. He could also be the one to accidentally push Cana and Gildarts closer together, like by getting them into a situation where they have to rely on each other. Lucy would also be a good addition. She's a good friend to Cana and can offer a different perspective on love and relationships, having her own experiences in the guild.
Billy Abbott could be an interesting addition. He could bring some outside drama, perhaps through a business deal or some personal vendetta against Adam. His flamboyant personality would contrast with Adam and Chelsea's relationship and add an element of chaos to the fanfiction.
Obviously, the main couple, the mysterious girlfriend and the male lead, are essential. But you could also include the male lead's friends. Their reactions to the girlfriend's mysterious ways can add some humor and depth to the story. For example, his best friend might be constantly trying to figure her out.
Splinter is another character that could be included. As their mentor, he can offer wisdom and guidance to Donatello and Leonardo. His influence can shape their relationship and how they deal with the challenges in the fanfiction.