Emil is one of the surviving characters from the original NieR. He returns in NieR: Automata as a traveling item shop on wheels. He is located near the Desert Zone entrance from the City Ruins: Center area. Shoot him once with your blaster to stop him and purchase items. Go to Emil's house near the City Ruins: Center crater. There is a large sewer pipe sticking out of the crater wall, with water flowing out of it. Jump inside and use the elevator to find Emil's house. Emil's house will only open after you have completed the "Lunar Tears" side quest. The solution to that side quest can be found under the "Easy "Lunar Tear" trophy". Loot the treasure chest in the final room of the black cave. Once you reach the house with all the Emil faces, take one of the masks on the display. Return to Emil and talk to him. You can find him driving around the Desert Zone entrance from the City Ruins: Center area. After exiting his shop, Emil will complain that someone is stealing from his house. Go back to his house a second time with 9S to find a treasure chest inside. Hack into it with 9S to take what is inside. As you leave, Emil will ambush you in the dark chamber outside his house to start the secret Boss battle. Emil is Level 99 -- so you must be high level to damage him. You also cannot access Emil's treasure without 9S -- so you will need to complete the game at least once to earn 9S and his hacking ability.