Kirby is the titular character and main protagonist of the video game series of the same name. It is unknown how old Kirby is, but it is implied that he is either a baby or a young child, or at the very least, has the mind of one.
While it is unknown where Kirby originated from, he has resided on Planet Popstar in the kingdom of Dream Land for the majority of his life. He is a small pink puffball with short, stubby arms and large feet. His main ability is that of inhaling his opponents and gaining their abilities in the process. Kirby has also appeared in all entries of the Super Smash Bros. series.
In M.U.G.E.N, there are various versions of Kirby, but the most well-known is Bane84's version, most likely due to it being able to change M.U.G.E.N's core gameplay into that of Super Smash Bros.'s when fighting against itself and characters that use the same base code.
Laxxe23 & Boss Lagomorph's version
This pink puff plays like Super Smash Bros. Melee's Kirby and uses custom-made sprites, though the rather imprecise positioning for said sprites causes the character to appear shaky during animations. Akin to Kirby's Copy Ability, inhaling an opponent grants Kirby a random powerup, though you shouldn't host creations that aren't yours.
Stated to be incomplete, this Kirby plays similarly to characters from the Marvel vs. Capcom series, complete with a Launcher and Super Jump. Perhaps it inhaled Tedhaun before the match, but Kirby occasionally comes back to life during simul mode, but only for short periods of time.
Seku's Kirby has a meter showing how many times it can puff up in midair (up to 5 times) and has various attacks from Kirby's Return to Dreamland, Super Smash Bros., Kirby 64: The Crystal Shards, and Kirby Air Ride. It can switch attack modes by eating its foe, but the meter (mentioned above) could cause M.U.G.E.N to lag. It also blocks like in Smash Bros. meaning that it can't block for too long, and can't block in mid air. Its A.I. is very easy.