After absorbing Monomon the Teacher, go to the Blue Lake (located in the crypt below the Resting Grounds, Desolate Dive {or Descending Dark} is required. to gain access*.
Go to the entrance near the crypt, and Quirrel will be sitting there. Talk to him, and after his dialogue is done you'll be prompted to sit next to him.
This will unlock the achievement.
*** Spoiler - click to reveal ***Afterwards, realize he's probably dead (based on his earlier dialogue from the City of Tears encounter) and mourn the loss of the best adventure boi there is.
*Alternatively, you can use the Monarch Wings at the alcove above Salubra's Shop and keep walking to find the Blue Lake.