The White Lotus sentries are some of the supporting characters in The Legend of Korra. They are lower ranking members of the Order of the White Lotus.
Following the conclusion of the Hundred Year War, the Order of the White Lotus decided it was time to reveal themselves to the world. Before Avatar Aang died in 153 AG, he instructed the Order to watch over his reincarnation and to assist them during their Avatar training.
In accordance with Aang's request, protecting and training the new Avatar became the organization's priority when the White Lotus leader and two other members discovered the new Avatar, a girl named Korra, at the Southern Water Tribe. The society created a compound dedicated to these objectives, and it was there where many sentries were stationed to closely watch over and to protect the Avatar.
After Avatar Korra escaped to Republic City to find Aang's son, Tenzin ordered her to return to the Southern Water Tribe. A small group of the Order's sentries arrived at Air Temple Island that night with the intention of returning to the South Pole with the Avatar immediately. Before the sentries could take Korra onto the ship, however, Tenzin reconsidered, and decided that Korra's airbending training should begin, thereby allowing her to stay on the island.
Many of the sentries were relocated to Air Temple Island after Korra's move, continuing to carry out Aang's death wish of protecting his reincarnation. They had a small building for themselves where they often listened to pro-bending matches via radio. The sentries were even present in the island's dining hall during a few of Korra's meals, making sure that the Avatar would remain on the island, a task they were not always successful in.
During the Equalists' attack on the air temple, the White Lotus sentries witnessed the arrival of the Equalist airships from their elevated quarters and rushed toward the main courtyard to confront the Equalist raiders. They launched a barrage of firebending, earthbending, and waterbending attacks, together with the help of Lin Beifong and Tenzin's children they eventually defeated the invaders. On orders from the former Chief of Police, they locked the captured Equalists up in the temple's basement. The tides quickly changed however when more airships made their way over from the mainland and deployed more Equalists on the island, prompting the residents to prepare for a quick escape. To allow Avatar Korra and her friends enough time to escape the island, four sentries remained behind to thwart and delay any possible pursuit of the Avatar by Equalists forces.
With the fall of Air Temple Island, a number of White Lotus sentries were captured and brought before Amon, who removed their bending abilities using his advanced bloodbending. It can be presumed that their bending was returned by Korra after Amon's defeat.
Five sentries came to bring food to a dangerous criminal and the leader of the Red lotus, Zaheer. However, Zaheer came airbending through the bars of his cell, and managed to defeat all the other sentries by throwing them into the cell. After he escaped, he managed to defeat all the other sentries and save his friends.
Meanwhile on air temple island, when Yoryu's true identity was exposed as Zaheer, two firebender sentries came to help Kya defeat Zaheer, but was easily defeated by the airbender, leaving Kya to fight him.
3 years later, the sentries became more responsible for Zaheer's incarceration, he was put in a prison loacted in the mountains. When Korra visited Zaheer, the sentries accompanied Korra in the elevator, before leaving Korra alone to face Zaheer. A few weeks later, sentries guarded Kuvira in her prison. The sentries were present when Kuvira is released from her cell.
The White Lotus sentries' uniforms are very similar to the uniforms worn by higher-ranking members of the Order with the main difference being that the sentries wear a helmet with the Order of the White Lotus' insignia on it, while higher-ranking members do not.
White Lotus Sentries on the Avatar wiki.