Master Hand

Master Hand (マスターハンド, Masutā Hando) is a white glove-like entity who acts as the creator of the Super Smash Bros. universe and is also the creator of the World of Trophies. He is the final boss of the Classic and Adventure mode in Super Smash Flash, and the final boss for the Classic mode in Super Smash Flash 2. Master Hand's origins are unknown, and his and Crazy Hand's purpose is undiscovered. But both hands are in a dualistic conflict of being creative and destructive; Master Hand and Crazy Hand respectively.

In Super Smash Flash
In Super Smash Flash, Master Hand is the final opponent of the Classic and Adventure mode, where he is accompanied in this by Crazy Hand. He always fights on the stage. He uses his size and shape to his advantage - he slaps, punches and flicks the player around.

Master Hand can't be defeated by normal means; instead, he has Heart Points (HP) which is altered based on the difficulty (for example, he only has 150 HP when fought on Very Easy mode) and must be depleted. Once his HP reaches zero, he explodes into the background and the mode is completed. In Adventure mode, once his HP reaches the halfway point, Crazy Hand will appear to aid Master Hand in the fight against the player.

In Super Smash Flash 2
✅ Master Hand was indirectly confirmed to be making some sort of appearance in Classic mode in SSF2, this is hinted by his appearance in the icon for Classic mode on the Solo menu screen.

Other appearances
Master Hand appears in the Game Boy Advance game Kirby & the Amazing Mirror as a mini-boss. Inhaling it would give Kirby the Smash ability, an ability that gives Kirby some of his moves from Super Smash Bros. Melee (though in a slightly altered state). Master Hand and Crazy Hand are also bosses of Candy Constellation.

However, the concept of the Hands may have come from a boss called Wham Bam Rock from Kirby Super Star and Kirby Super Star Ultra (the game's remake), who attacked Kirby with a stone hand that used different slaps, pokes, and grabs. In the updated version, Kirby Super Star Ultra, a crystalline version called Wham Bam Jewel appears, with more parallels to the two hands.