Hasumi Seika

The older of the sisters next door. A nurse. A cheerful, lively, caring and supportive older sister type.

She dotes on the protagonist, whom she has always viewed as a little brother, a little too much. When the protagonist receives a love letter, she realizes that she sees the protagonist as more than just a little brother. Thinking that he might leave her for someone else if things continue as they are, she uses the excuse of "helping him with his studies" to pursue him sexually at every chance.