Nekketsu Kouha Kunio-Kun created the brawler genre with its revolutionary combat system and urban aesthetics. Except for co-op, it's got everything that make beat 'em ups great: many simultaneous opponents, combos, back attacks, throws, jump kicks, and even ground attacks.
Kunio's friend Hiroshi keeps getting beaten up by rival gangs. It's up to hot-blooded tough-guy Kunio to avenge his friend by beating the snot out them one by one.
Nekketsu Kōha Kunio-kun would spawn several spin-offs, such as the Downtown Nekketsu Monogatari (River City Ransom) games and a bunch of sports-themed minigame titles. Most of these were localized out West but not under the Kunio license. Some examples include Nintendo World Cup on Game Boy and Crash 'n' The Boys on NES.
"Nekketsu" is Japanese for "hot-blooded." It's the name of Kunio's high school as well as a description of his character. "Kōha" means "tough guy."
Double Dragon II, also from Technōs, uses this game's unique right-left attacking system. However, in that game you can freely face left or right so you can use your powerful back kick all the time.
This is the only arcade brawler that is single player only. Besides that, it has a remarkable number of features that brawlers to this day still use.
Nekketsu Kouha Kunio-kun started the genre off on the right foot. It was way ahead of its time with its visceral violence, tough enemies, and huge moveset. Its combat definitely takes some getting used to, but it's extremely satisfying once mastered. Its graphics are fairly ugly, but its music makes up for it by being super catchy and memorable.