The Ha-Ha Brothers: Kazuki Jūmonji (十文字 一輝, Jūmonji Kazuki?), Koji Kuroki (黒木 浩二, Kuroki Kōji?) and Shozo Togano (戸叶 庄三, Toganō Shōzō?), are fictional characters of Eyeshield 21. Though constantly re-affirming that they are not brothers, the Ha-Ha Brothers give that impression since they are constantly together. They are so called due to their schtick of saying "Ha?" in rapid succession with a crescendo (rising volume) in response to a challenge or something they disbelieve. This 'routine' starts with Togano and ends with Kuroki. (Ex. Togano: HA?! Jumonji: HAA?!! Kuroki: HAAA?!!!) The Ha-Ha Brothers consist of three delinquents; "Ha-Ha Brothers" is even written in English in the anime and manga when they are VS somebody, along with all the other characters names.

Source: Wikipedia