William Peter Hjelte, better known by his gamer tag Leffen, is a Swedish professional fighting game player and streamer. Although mostly known as a Super Smash Bros. Melee player, he has also competed in Dragon Ball FighterZ, Guilty Gear Strive, and Super Smash Bros. Ultimate. In Melee, Hjelte plays the character Fox, and is considered one of the best players in the world, having been ranked as one of the top seven Melee players in the world every year since 2014. A 2021 list compiled by PGStats ranked Hjelte as the sixth greatest Melee player of all time.

Professional Super Smash Bros. competition involves professional gamers competing in the Super Smash Bros. series of crossover fighting games published by Nintendo. Organized tournament competition began in 2002 with Super Smash Bros. Melee, released for the GameCube in 2001; however, in the series' native Japan, there have been tournaments as early as 1999 with the original Super Smash Bros. for the Nintendo 64. Later tournaments have featured the other games in the series, with the two largest and most popular Smash Bros. scenes revolving around Melee and Super Smash Bros. Ultimate for the Nintendo Switch. Smaller scenes exist for the original game and Project M, a popular fan modification of Super Smash Bros. Brawl for the Wii, and to a lesser extent, Brawl itself. Major Smash Bros. tournaments include the GENESIS, Evolution Championship Series (EVO), Super Smash Con and The Big House annual series. Major League Gaming (MLG) has also previously included Smash Bros. games in its Pro Circuit.