The Gatekeeper, full name Ares Teena Dloe Gynosan P. Ruporson Gear Amadeus 5th (アレスティーナ ドロエ＝ギョナサン Ｐ ルーボソン ギア アマデウス ５号 ; Aresutīna = doroe = gyonasan = P = rūboson = gia = amadeusu 5-gō ) is the sentry of the former European Branch Headquarters of the Black Order.
The Gatekeeper is, quite literally, a gate; he's a giant face located between the two gates in the large wall that surrounds the former European Branch Headquarters.
The Gatekeeper is extremely cowardly and appears to be somewhat mysophobic, screaming out in agony when Allen Walker touched him because he did not want to touch someone who was cursed.
Though his actual age is not known, the Gatekeeper has been serving as the guardian of the European Branch for sixty-five years, and is the fifth Gatekeeper.
The Gatekeeper is first seen when Allen Walker makes his way to the Black Order European Branch Headquarters, sitting silently as Allen introduces himself to those in the security room via golem. After Allen is told to take the Gatekeepers' sensor exam, the Gatekeeper hesitates before quickly leaning forward, shocking Allen. Lights flash out of his eyes as he stares at Allen, and when Allen's image initially looks fuzzy, the Gatekeeper wonders why he can't read him.
Allen's cursed eye, though, was read, and as soon as the Gatekeeper notices it he declares that Allen has failed the test and activates the building's alarm system, which catches the attention of Yu Kanda. Knowing the cursed pentacle was the mark of an Akuma, the Gateeeper deduces that Allen was one of the Millennium Earl's henchmen, shocking Allen and the members of the Science Division who are in the security camera room. By the time Kanda arrives, the Gatekeeper is crying.
After Kanda sees Allen's Anti-Akuma Weapon, he snaps at the Gatekeeper, who reasons that they can't really tell without seeing his insides. Allen throws himself at the Gatekeeper, begging him to believe him, but the Gatekeeper is revolted and starts crying again.
Kanda, influenced by the Gatekeepers reasoning, decides to cut Allen open with his Anti-Akuma Weapon, Mugen, so another examination can be performed. As soon as Allen mentions his master, General Cross Marian, though, Kanda stops, and a letter sent by Cross is found. After this, Komui Lee gives the order for Allen to be permitted entrance, and Reever Wenhamm relays the orders to the Gatekeeper to open the gates.
Though the Gatekeeper is reluctant, he complies.
Disappearance of Cross Marian Arc
The gatekeeper could not be transported during the move to the new headquarters so he had to be deactivated. A piece of it was kept and is now decorating the Science division room. 
Powers and Abilities
Akuma Sensor: The Gatekeeper has been designed to act as an Akuma sensor, scanning all who wish to enter the European Branch to make sure their skeleton is not mechanical.
Remote Access of the Building Alarm System: When the Gatekeeper noticed Allen's cursed pentacle on his forehead, he was able to take over the Headquarters intercom system and activated the alarm, alerting the rest of the building (including Yu Kanda) that there was an 'intruder'.
- The Gatekeeper likes to spend his time observing humans and cute girls, but despises cursed humans (i.e. Allen Walker).