She. Is. A. Minor.
Is she, though? The only reference I can find to her age is that she's in high school, but I turned 18 when I was still in high school so it's certainly possible she is. Masumi does mention being "an adult" and calls Satoko a kid but that could just be how she views their relationship rather than being literal. The age of consent where they live might be lower than 18 anyway.