Yumiko is the secret antagonist and obsessed lover over sumiko. She is first seen as emotionless and dead but once sumiko is around she acts girly and innocent. I imagine her voice to be quite middle to little bit high pitched and a little monotone.

The first line,her laugh, would be a normal girl laughing but with a bit of an edge to it(that chill that goes up your spine)

The second line is sweet sounding and awkward. Yumiko comes off as nice and understanding.

The third line she is quiet and monotone at the start of her sentence turning into a shout of anger mixed with determination.