Early in the Mario franchise's release history, Nintendo released the Game Boy platformer Super Mario Land in mid-1989, in which Mario traveled to a far-off land to rescue Princess Daisy. This gave way to a subseries of handheld platformers, and the first sequel, Super Mario Land 2: 6 Golden Coins for the Game Boy in late 1992, debuted a new antagonist that resembled a larger, portlier, more thuggish counterpart to Mario, Wario, introduced as one of Mario's previously-unseen enemies. Wario, originally presented in a more straightforward, villainous light, takes over Mario's castle while Mario is away, and when Mario returns he must go on a quest to collect the six titular coins that are the keys to his castle in order to defeat Wario and reclaim it from him. Wario, for a time, then became one of few recurring Mario villains that were not Mario's traditional arch-enemy Bowser, and was reused as such in several puzzle games:Mario & Wario (a Japan-only release for Super Famicom in 1993) and Wario's Woods (released in the West in December 1995) both featured him enacting revenge plots against Mario and his friends, and in one of Nintendo's first crossovers with a third-party franchise, Wario Blast: Featuring Bomberman, a Game Boy installment of the Hudson Soft franchise Bomberman, the eponymous hero must prevent Wario from plundering his home planet.

Wario was then permanently assigned a new role and outlook in the Mario universe, that of a less-malicious anti-hero motivated by a comically insatiable greed. Like Yoshi and his own relevance in the Mario games, Wario forever became part of the stable of recurring Mario side characters that would take part in a variety of Mario games, such as being a selectable racer in Mario Kart and a selectable "board piece" in Mario Party, while at the same time beginning to star in games all to himself. The "third" game in the Super Mario Land subseries, Wario Land: Super Mario Land 3, became the first game in the Wario Landseries of platforming titles for various platforms, a series which established a style of platforming different from the familiar Mario formula of jumping and bouncing on enemies and focusing more on physical attacks such as running tackles. The scenarios in Wario platforming games typically center around Wario's greed-motivated adventures and the incidental deeds of goodness he commits for others while doing so, such as defeating a more threatening villain in his quest to claim the reward money. Early in his own games, Wario briefly had an equally greedy rival of his own, a female pirate named Captain Syrup, who had only recently made a return appearance in 2008's Wario Land: Shake It! (the most recent Wario-centric platformer) after a decade of absence.

Starting from 2003, Wario additionally and regularly starred in a more deranged, parodic, borderline scatological series of party games titled WarioWare, with an all-new cast of bizarre side characters and a very different twist on the established party game formula. In these games, the scenario is that Wario gets the idea to strike it rich by founding his own video game company and getting all of his friends from his home city, Diamond City, to program hundreds of extremely simple games for him. The gameplay itself resembles an extended, rapid barrage of extremely simple "games", each lasting mere seconds and taking no more than one or several appropriately timed button presses to complete ("Microgames"), and the number of microgames a player can complete before failing a set number of times is set as their high score. There is seemingly no limit to the range of surreal imagery the microgames display; highlights include Wario having to jump at the right time in order to avoid getting run over by a giant hot dog on wheels, reenactments of classic scenes from old Nintendo games, and successfully guiding a finger into a nostril. Different installments of the series for different platforms have featured their own, unique twists to the gameplay dependent on the hardware of the console itself.



9-Volt is a major human character in the WarioWare series. He is a Nintendo fan, he collects everything about this company, and loves to play video games. 9-Volt lives with his mother, 5-Volt, and his pet Fronk in their house in Diamond City and attends Diamond Elementary. He debuts in WarioWare, Inc.: Mega Microgame$! as one of Wario's friends who helps him produce microgames.

Since WarioWare: Twisted!, 9-Volt always stars together with 18-Volt in the WarioWare series, who becomes one of his best friends in this game.