Former Watcher!Mumbo anyone?
The others didn't know why Grian grew so attached to Mumbo at the start of that first season. Some just thought it was Grian looking for someone to show him the ropes and the man of spoons gladly helped him.
To Grian, he had found his protector. The one person who helped him when they were stuck with them. How scared he was when Mumbo got his wings taken and thrown out of that place, leaving Grian with them.
Grian wouldn't let anyone hurt his protector. He promised Mumbo, promised that he would find him and protect him against them just like he had done for him.
It didn't matter to Grian that it seemed that Mumbo has completely forgotten who he was. Forgot the countless years under the thumb of them.
Grian would stay close by, would make sure that the purple outline of his long gone wings would not return. Wouldn't let Mumbo be brought back under their control.
But even Grian could still feel their hands on his neck, whispering what they wanted from him. He was sent to observe and make this place ready for them, for their arrival. He has no time to make friends, only companions who would be loyal to them.
Maybe it would be worth it, to hang out with Mumbo. To be the Watcher of Hermitcraft. Just like they wanted.
But he was Grian, one pesky bird. He takes the rules into his own hands, than you very much.