Master Hand is the creator of the Super Smash Bros. universe. Despite always being the final boss, he isn't much of a bad guy; this is a bit easier to see in The Subspace Emissary and The World of Light, and also by the aforementioned fact that he created the SSB universe, but has never destroyed it.
Master Hand is respected by most of the characters, but there are a few exceptions (such as Ganondorf). He likes to create, whereas a similar character likes to destroy, although still never destroyed the SSB universe; however this might be because they are friends, and Master Hand doesn't want Crazy Hand to destroy his creation, but this hasn't been confirmed, nor has any explanation.
Master Hand's Brawl Trophy implies that he doesn't use his full power on the characters, and just toys with the characters that have many victories for fun; some people think that his full power might be Master Core, but the true relation between the two is unknown. In Super Smash Bros. Ultimate, prior to the events of World of Light the evil fiend, Galeem, Lord of Light, captures Master Hand and uses this to create an army of false Master Hands in his plot to drown the entire universe in light. Thanks to Kirby, who manages to escape the beams of light that Galeem fires out, the fighters and spirits are gradually rescued.
In "The Final Battle", the true Master Hand is eventually defeated and freed from Galeem's control. Once the true Crazy Hand (who was captured by Dharkon) is freed, the two open a rift where Master Hand enters to defeat 50 fighters of light and darkness; this marks the first official time Master Hand is a playable character aside from the use of glitches or cheating devices in past games.
Master Hand on the Villains Wiki