The hermits know he's a watcher. There was really no point in him hiding it because everyone knows that not all watchers are evil, in fact most aren't evil.

Watchers, according to local legends, are godly beings that watch over servers to make sure nothing bad happens. It just so happened that the watcher watching over EVO got Jealous of the friendship between the members. After most of her plans to tear them apart didn't work, she decided that she would target the admin, which so happened to be Grian.

Watchers are either born or made. Grian, unfortunately was made to go though the process of being turned into a watcher....in front of the other EVO members. Her plan didn't go as she would like because instead of going out of control, Grian ended up killing the watcher because she threatened his friends.

Grian found himself being taken into the void after the events by the Queen of the Watchers, Ruby. She told him that he could stay and train under her or be turned back to normal. Grian ended up choosing to train under ruby, he thought, what if another watcher decided to attack his friends, he needed some way to protect them.

And that ended up happening during season seven, when he told them he was a watcher.

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Sorry this head-cannon is kinda long- but I really like this idea about watchers and stuff!