Risa's younger brother and main character in the Lovely Complex Deluxe one shot and Lovely Complex Two.

Love is unusual for Koizumi Risa and Ootani Atsushi, who are both striving to find their ideal partner in high school—172 cm tall Koizumi is much taller than the average girl, and Ootani is much shorter than the average guy at 156 cm. To add to their plights, their crushes fall in love with each other, leaving Koizumi and Ootani comically flustered and heartbroken. To make matters worse, they're even labeled as a comedy duo by their homeroom teacher due to their personalities and the stark difference in their heights, and their classmates even think of their arguments as sketches. 
 
Lovely★Complex follows Koizumi and Ootani as they encourage each other in finding love and become close friends. Apart from their ridiculous antics, they soon find out an unexpected similarity in their music and fashion tastes. Maybe they possess a chemistry yet unknown, but could love ever bloom between the mismatched pair? 
 
(Source: MAL Rewrite)

Rather than bow to the inevitable, Risa and Atsushi join forces to pursue their true objects of affection. But will their budding friendship become something more complex?
 
(Source: Viz Media)
 
Note: The chapter count includes the one-shots "Hoshi ni Nattemo Aishiteru" and "Bokura no Ibasho".

The original romantic comedy followed two close, but bickering, high school friends with height complexes: a tall girl named Risa Koizumi and a short boy named Atsushi Ohtani. The advertising blurb for Love&#9733;Com D promises that "the family of Love&#9733;Com will return," but hints that Risa and Atsushi will not be main leads this time. 
(Source: Anime News Network) 
Included one-shot: Honey Going

Takahito Koizumi