LISA: The Joyful - Definitive Edition

In the W2D worshipper's cave, go left to collect a secret green helmet. Before finishing the list, head to the Joy Boys Hideout and talk to the man outside of it without your mask. Return to this location after finishing the list to unlock the achievement.

(Above excerpt copied directly from the Fandom Lisa RPG Wiki, https://lisa-rpg.fandom.com/wiki/Steam_Achievements)



Note that the Joy Boy's Hideout is the location where you obtain the Joy Mask, the man you need to interact with is outside the hideout and has orange hair and wears a green outfit.

See my guide posted under the "The Beat Never Dies" achievement for more details on accessing the W2D Worshipper's cave.

See my guide posted under the "Joy" achievement for more details on accessing the Joy Boy's Hideout.