Minami-kun ga Koibito!?

Chiyomi is a high school girl, who was born and raised in a retro Showa-era shopping district in Shonan. She is a member of the basketball club and has a kind personality that stays cheerful despite her somewhat complicated family structure. She and Minami-kun, also known as Minami Hiroyuki, a basketball star from her hometown of Shonan, have been like siblings since childhood, and they are happily in a relationship. However, one day, he suddenly becomes 15 cm in size and fits in the palm of her hand. Chiyomi begins living with him in his now tiny body.
(Source: Japanese = Yahoo Japan || Translation = MDL)
~~ Adapted from the manga series "Minami-kun no Koibito" (南くんの恋人) by Uchida Shungicu (内田春菊).