How to Get Fawkes As a Companion in Fallout 3

Stock up on weapons and medical supplies. You will need precision guns, at least one heavy weapon, lots of Stimpaks, and heavy armor.

Travel to Little Lamplight. There are two ways to get into Vault 87 from there: you can either go through "Murder Pass" (a cave filled with aggressive Super Mutants) or go in through the back door, an option only available if you have 50 Science.

Kill the Super Mutants as you progress through. Rest when you can.

Go deeper into the vault. Feel free to loot as you clear out the Super Mutants.

Find the vault isolation lab. Fawkes will be in room 5.

Talk to Fawkes through a speaker. He will ask you to set him free. Activate the fire alarm, or pick the lock on the door.

Talk to Fawkes again. He will offer to find you the G.E.C.K. Accept his offer.

Follow Fawkes deeper into the vault. Wait for him to enter the irradiated lab and retrieve the G.E.C.K.

Leave once you have the G.E.C.K. You will be ambushed by Enclave. Don't use cheats to leave, as you need to be in a certain location to trigger the Enclave ambush. Using cheats to leave will break the main quest, creating a bug that can only be fixed by returning to Vault 87.

Break from containment. Speak to President Eden, then leave.

Exit out to Raven Rock. Fawkes will be waiting with a Gatling laser.

Fawkes will request to join your side if you have Good Karma. Accept him. If you don't yet have Good Karma, don't worry. You can go back and complete side quests in altruistic ways and then return to Fawkes, although he may be waiting in the Museum of American History.