My idea for Usagi to get into ROTTMNT: Hamato and Usagi were sister clans before humans drove off Yokai. Being yokai, they taught the Hamato how to harness mystic energy to create the Hamato Ninpo. Miyamoto helped seal away Shredder cause someone had to close the door to the Twilight Realm after Karai dragged him in there with her.
Yuichi is the modern day Usagi who’s living in Japan. But when the Shredder and Karai are released from the Twilight Realm, the spirit of Miyamoto reaches out to him telling him to find the Hamato clan and reseal the Shredder. But the Turtles and family beat Shredder and freed Saki’s spirit before he even got to America. So he just shows up ready to smack down only to be told it already happened, and now he’s just there in this huge ass city he doesn’t know how to navigate cause he’s from the yokai sticks of Japan. But still decides to stick around cause these four turtles and the rat are the famous Hamato clan his family has told him about (he could’ve sworn Auntie told him they were humans though) and maybe they could help him train to become a legendary samurai! (not only do they seem to know what they’re doing but they’re also fun to be around plus that one with the stripes is kinda cute)
