The hidden boss in Super Smash Bros. Melee’s Adventure Mode is Giga Bowser.
Giga Bowser appears as a secret final boss after the normal Bowser battle on the Final Destination stage. To encounter him, players must reach the end of Adventure Mode on Normal difficulty or higher and complete the earlier stages within 18 minutes without using a continue. These conditions make the fight a hidden reward for fast, consistent play rather than a standard ending.
Giga Bowser is a gigantic, heavily empowered version of Bowser with enormous reach, strong attacks, and powerful resistance to being launched. He first appeared in Melee and later became closely associated with Bowser’s Final Smash in subsequent Smash games. In Melee, however, he is not simply an alternate costume or ordinary playable fighter; he functions as a boss character.
Master Hand is the usual final opponent in Classic Mode, while Crazy Hand is another hidden boss associated with that mode. Ridley appears as an opponent in Melee but is not the hidden Adventure Mode boss.