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.
The coach could be a great addition. He could be the one who gives Kazemaru and Endou special training sessions or advice that helps them grow as players and also strengthens their bond. And perhaps some of their opponents too. For instance, a rival team's star player who challenges Kazemaru and Endou both on and off the field, making them strive harder to improve.
Well, obviously Naruto and Hinata should be included. Naruto as the father can give some wise advice to Boruto and Himawari. Hinata as the mother can show her love and care for them. Also, Shikadai could be a great addition. He is Boruto's friend and his calm and intelligent nature can contrast with Boruto's more impulsive side, and he can also interact with Himawari in an interesting way.
Definitely Mary Jane. She could be in a dangerous situation that forces Spider - Man and Venom to work together. Her relationship with Spider - Man adds an emotional depth to the story.
Toad could be a character in the fanfiction. He could be the messenger or the one who sets up the initial situation that brings Wario and Waluigi together. Captain Toad could also be included. His treasure - hunting skills could be useful in a story where Wario and Waluigi are on a quest for something valuable. And Rosalina could make an appearance, perhaps as a wise figure who gives advice to Wario and Waluigi during their adventures or disputes.
In addition to Naruto, Sakura and Sarada, Kakashi could be a great addition. He has a long - standing relationship with Naruto and Sakura and could serve as a mentor figure to Sarada. His wisdom and combat skills could add an interesting dynamic 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.
Obviously Ratchet and Clank themselves are essential. But adding characters like Captain Qwark can add some humor. He could be the bumbling hero who sometimes gets in the way but also has his moments of usefulness. Another good character to include could be Dr. Nefarious. His evil plans can create great conflict for Ratchet and Clank to overcome.
Sigma should be included. He is a classic antagonist in the Mega Man series. His presence could create a situation where Bass and X are forced to put aside their differences. Alia would also be a good addition. She could provide intelligence and support to X and Bass, and her interactions with Bass could be interesting, especially if she tries to understand his more chaotic nature.
Axl can be a great addition. He has his own unique abilities and a bit of a wilder personality compared to X and Zero, which can add some interesting dynamics. Also, Sigma is often included as a major antagonist. His complex motives and powerful presence make for great story material. Another good character is Alia. She can provide support to X and Zero from the base, and her intelligence can be used in various ways in the fanfiction, like helping to analyze enemy weaknesses.