5-Volt is the mother of 9-Volt and a recurring secondary character in the WarioWare series. She made her first appearance in WarioWare: Twisted, but her face wasn't shown until Game & Wario.
In Game & Wario, 5-Volt takes an active role in 9-Volt's minigame, "Gamer." As 9-Volt secretly plays video games in bed, she will occasionally appear in a glowing-eyed silhouette, attempting to catch 9-Volt in the act. 9-Volt must therefore pretend to be asleep whenever she appears. Though the game portrays her as comically demonic, at times emerging from 9-Volt's television in her attempts to catch him, the game is labeled with a humorous disclaimer stating that 5-Volt isn't actually as scary as the game portrays.
Otherwise, 5-Volt is depicted as a mother that urges her son and his friend 18-Volt to take care of their chores, but otherwise is OK with their video game habits. She's also shown to be a skilled gamer herself, as depicted in Game & Wario; she takes 9-Volt's game away when she orders him to do his homework. When 9-Volt finishes and gets his game back, he's shocked to discover that she's blown his high score out of the water.
Super Smash Bros.
In Super Smash Bros. for Wii U, there is a stage based on Gamer from Game & Wario. As the fighters duke it out, they need to be aware of 5-Volt, who will deliver her own punishment to anyone caught in her gaze.