It's the if line on the ten-year side
That is to say, Arisu did not move and did not become a cub with Tsuna. He did not meet Tsuna until he was in junior high school.
Please read Chapter 1 for details.
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Before writing it, I decided that it would be "be", but when I started writing it, it seemed weird. Maybe Arisu really likes Tsuna. In that case, maybe it's still "he"?
Intention: Before experiencing anything, you have already become yourself