Legion is a fan-favorite character in Mass Effect 2. The helpful Geth can even be added to your crew on the Normandy. Legion is actually 1000 different Geth entities that have been combined into a singular robot, and he can be a fantastic addition to the ship, from a narrative point of view.
Legion can be found in the Reaper IFF mission. Players will be send on the hunt for an IFF object that will grant them passage through the Omega 4 Relay. While exploring a derelict Reaper and searching for the IFF object, players will encounter Legion.
They will pop up from time to time, helping out the player before eventually being knocked unconscious. Players can then decide to bring Legion back to the Normandy and reactive it, or sell it to Cerberus instead.
Obviously, players who wish to add Legion to their crew will need to reactivate the robot instead of selling it. Once Legion is part of your crew you can upgrade them, take them on missions, and even unlock a loyalty mission for them, the same as everyone else on the team.
Legion is easily the most unique crew member that players will have access to in Mass Effect 2, so it is strongly advised to add them to the crew instead of selling them.