You will need to have completed more than half of the game to be able to earn this achievement.
Mister Little is in the Graveyard connected to Mimiga Village, he's easier to spot if you clear the area of monsters and look for movement on the bottom floor of the Graveyard.
If you speak to Mister Little and are far enough along in the game he will ask you if you will take him home.
Say yes and you will collect him as an item.
Mister little's home is tucked away at the bottom of the area you enter after the Egg-00 room- be careful it is very easy to die without using
an item you collect during the course of the game.