from The Results From When I Time Leaped to My Second Year of High School and Confessed to the Teacher I Liked at the Time (Manga)
Natsumi is Haruka's younger sister. She is still in high school and gets along really well with her sister, approving her Relationship with Seiji (MC) regardless of him being her student or their age difference.

The Results From When I Time Leaped to My Second Year of High School and Confessed to the Teacher I Liked at the Time (Manga)
I was a corporate worker in my 30’s, when I woke up and found that I had time leaped to the spring of my second year of high school. At the time, there was a teacher that I had regretted not confessing to, so I confessed to her in the heat of the moment, and now we’re dating! However, the teacher is one who would make any man a good for nothing person, so I end up being spoiled by her a lot. While secretly dating my overworked teacher (lover), a sweet school life filled with flirting awaits. A 2nd year high schooler with a 30 year old me on the inside and the teacher (lover) that I want to pamper, our everyday romantic comedy begins.

The Results From When I Time Leaped to My Second Year of High School and Confessed to the Teacher I Liked at the Time (Light Novel)
I was a corporate worker in my 30’s, when I woke up and found that I had time leaped to the spring of my second year of high school. At the time, there was a teacher that I had regretted not confessing to, so I confessed to her in the heat of the moment, and now we’re dating! However, the teacher is one who would make any man a good for nothing person, so I end up being spoiled by her a lot. While secretly dating my overworked teacher (lover), a sweet school life filled with flirting awaits. A 2nd year high schooler with a 30 year old me on the inside and the teacher (lover) that I want to pamper, our everyday romantic comedy begins.