Theo Stoker is a blogger who writes about the occult on The Bog Witch. He made his debut as a 'writer' in The Snarky Cats of Ulthar blog with The Heavenly Journey of Brother Lucas (that has since been moved to The Indie Cats of Ulthar blog), which offered a glimpse into the Dreamlands. In the end of the post there was an update made by someone else that revealed that Theo had disappeared after going to meet someone named Bob... When in the current timeline did this happen is unclear.
He sometimes get carried away with his theorizing and is always looking for lesser known folklore. He's a friend of Nikki, the daughter of Tim's mentor, Ben, and his first appearance in the series was in Tim's Winter Adventure, where he told them about Krampus and his creepy postcards.
Theo's mother, Dr Joanne Stoker, showed up leading a group therapy session in Penny and the Silver Key Part 1.