LISA: The Joyful - Definitive Edition

Finishing the game with The Source achievement requirements.

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



Note that meeting the requirements for this achievement locks you out of obtaining the "Yado" achievement; using multiple save files is recommended.

Also note that the final choice to "Leave them" or "Join them" does not effect this achievement.

This achievement is rewarded after viewing unique post-credits dialogue between
The Joy Lab must be visited to trigger this dialogue to appear. See my guide on "The Source" for further details.