Shouta Tanaka's father recently remarried, and to take advantage of their honeymoon, he leaves his son in the hands of his new wife's daughter, Kaoru. This begins a new cohabitation between the young Shouta and the impassive Kaoru, whom he particularly likes.
Minus Ondo no Serenade is a completed manga by Ichikawa Natsuwo, first published in 2016. It blends romance, comedy and slice of life. You can read all 3 available chapters of Minus Ondo no Serenade online for free on MangaPulse; the newest is Chapter 3, added 2020-09-01. The series is finished, so you can read it from start to end in one go.