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The following is a draft
Kusuo Saiki of the second dimension - reincarnated into Qi Munan of the third dimension (an ordinary person who was happy at the beginning of school suddenly died and then traveled through time) - Qi Mufengyue who traveled back to the second dimension (because he did not exist, his appearance was rationalized by the world as a Yokohama orphan) - restored super powers (more or less guessed what happened in his previous life because he read some comics)
Reincarnation (reason) + time travel + gradual recovery of superpowers (in urgent need of a suppressor): Qi Munan
Fluctuation of time and space (the reason for the fusion of the world) + after the if line plot: Saiki Kongsuke
Summary: The two people were originally separated by yin and yang (?), But because of the lucky roll of the dice in the book (some kind of unexplainable fate), they got together with some incredible and extremely small probability.
The final outcome: I will always stay and protect this world with a HE ending for all members. The world will merge a lot until Qi Munan's soul quality is enough to be stable in this world.
Ps: I just want to give my brother in the if line a good ending (~)
Concept: sweet love VS paralysis and tsundere