Makoto meets a sweet, shy girl named Haruna helping out at the school's athletics festival. He confesses his feelings for her the next time he sees her, and she bluntly rejects him, only to call him later and ask him out herself! The source of all these mixed signals is Haruna's "evil" twin sister, Natsumi. Natsumi is the opposite of Haruna, she's a forward, crude girl bent on trolling the hell out of her twin sister. Makoto is then stuck in the shenanigans between the two sisters, especially when Haruna shows that she's not one to be trifled with.
Haru to Natsu is a completed manga by Takeda Sun, first published in 2008. It blends romance and comedy. You can read all 29 available chapters of Haru to Natsu online for free on MangaPulse; the newest is Chapter 28, added 2018-02-05. The series is finished, so you can read it from start to end in one go.