The Drifter is an NPC in Destiny 2 that can be found in the Tower. He is not located in the main part of the Tower, and can instead be found in the Tower Annex. You have two options to get there, you can either spawn in the usual place and cross the map, or can spawn in at the Tower Annex, and the Drifter will be directly ahead on your right.
Open up the map in the Director and place a waypoint on the Drifter, show in the image above, to get a little in-game waypoint that will guide you to him.
Who is the Drifter?
If you are searching for the Drifter, then a little bit of information about who he is might be helpful. The Drifter is the character who oversees modes like Gambit and The Reckoning. Gambit is a mix of PvE and PvP, where your team takes on hordes of enemies, while also invading and being invaded by the opposite team.
Reckoning is a more straight forward horde mode, where you take on waves of increasingly difficult enemies in the search for loot. He will give out bounties and quests each week, and is tied into many of the game Exotic quests.
The Drifter is an interesting character, with a surprisingly rich and deep lore. He is also quite hard to get a read on, and it is difficult to know if he is a hero or a villain, or what exactly his long term plans are. Drifter brings a welcome dose of moral ambiguity to a game that is often very black and white. If you are interested in learning all about the Drifter, we would suggest the lore article on Destinypedia.