Yocchan is a non-canon character from the One-shot. She is Mina's lover.
Theories
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Shimeji on Shimeji's head are the Yocchan. She wants to become a mushroom. She dies on the same day. In the end of the one-shot Sis tells Mina she wants to make a Yocchan 2.0 — u/bouncepaw 2020-11-21
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sounds stupid, though. And I'm lost, who's who? Now we have both Sis and Mina in the one-shot? — u/drcheems
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Shimeji on Shimeji's head are the container for Yocchan. For her spirit or something — u/bouncepaw 2020-11-21