I headcanon that Hilda occasionally visits the Bell Keeper at his post to chat with him in-between the season 2 episodes. They talk about their shared interests in trolls and woffs, and their shared dislike of Erik Ahlburg. I also like to think that the Bell Keeper is Johanna's cousin (and therefore Hilda's uncle).
I like to imagine that Hilda and the Bell Keeper gossip about different creatures, especially the Vitra. They've both had run ins with the Vitra with varying levels of success. The Bell Keeper upset them 10 years ago and has been trying to win them over ever since. The Vitra actually quite like him at this stage but they don't want to admit to it. Hilda and her friends showing up and messing up their hibernation gave them the perfect excuse to start targetting some one else. But of course with the tunnels being filled in by safety patrol the Vitra have a new enemy that they share with both Hilda and the Bell Keeper.
Aside from Vitra, Hilda and the Bell Keeper also like to talk about Trolls, with both of them having lot of fear and respect for them. The Bell Keeper doesn't want to reveal the true extent of troll activity to Hilda because he doesn't want her to put herself in danger.
Their favourite topic of conversation though is Woffs. The Bell Keeper is an expert on Woffs and knows more about them than probably anyone else. Hilda has loved them since she was little because she's had a stuffed toy Woff for years (she doesn't know that it was the Bell Keeper who sent it to her for her sixth birthday, she just knows some unknown uncle sent it). She is especially interested in White Woffs since she learned that Frida is a witch, and that they carry special meaning for them.
The Bell Keeper is Johanna's cousin but she has no brothers so she calls him Hilda's uncle. He is one of the only family members she still has semi regular contact with. He has sent Hilda gifts for years but Hilda has never (to her knowledge) met him.
One thing Hilda does know however is that both her and the Bell Keeper think that Erik Ahlberg is a complete idiot. They love exchanging stories of his incompetence and often share a cup of peppermint tea over such stories.
