He's the movie's protagonist—a genuinely kind and brave guy who's also quick to get scared or discouraged, which makes him feel pretty human. The whole film plays up his crush on Tanya; he's constantly drawing her, and it's pretty obvious he's into her. Things get messy between them when his father disappears. They argue about whether Tanya told her dad about a supply cave they'd discovered earlier that same day, and they split on bad terms. They never see each other again after that.
Later, when Junpei has to deal with his brother's illness, he keeps coming back to this idea of a galactic train—a concept that becomes his way of processing life and death by the film's end.
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