Several weeks have passed since Aster's encounter with Mikasi, and he is now permitted to study witchcraft with the girls. But he has a lot to catch up on, and a formidable task ahead of him is in the curing of his estranged grand-uncle-turned-monster Mikasi. Aster is reluctant to face him again, and soon discovers he's not the only one in his family with lingering fear from Mikasi's attacks.
Meanwhile in the non-magical world, Charlie meets a new student, Ariel, a loner who's suspicious of everyone. Charlie befriends Ariel and tries to coax out her kinder side, but it turns out to be a greater challenge than Charlie expects when she learns that Ariel is under the thrall of a malevolent spirit known as a Fetch.
The comic was followed by The Midwinter Witch in November 2019, concluding the series.
Tropes found in The Hidden Witch: (spoilers for The Witch Boy are unmarked)
- Ambiguously Gay: Charlie is noticeably put off when two of her girl classmates excitedly gossip about a boy crush.
- Ambiguous Situation: While the characters do wonder how a witch ended up growing up outside the magical community, Ariel's origins are never fully explained, and that's considered less important than trying to give her guidance and support now that they know she exists. However, this plot thread of where she came from is picked up in The Midwinter Witch.
- Anti-Villain: Ariel, the new student at Sterling Junior High. Due to frequent bullying at her previous schools, her magical ability has begun to fester into forming a sadistic dark spirit. It obeys her wishes to harm or help, but grows stronger the more it's used and eventually will kill her if it isn't stopped.
- Casting a Shadow: Ariel's Fetch manifests as a shadow resembling her. She thinks of it as a Living Shadow, but it's not actually tied to her and gets more independent and less human-like the stronger it gets.
- Containment Field: After the events of the first book, Mikasi is being kept in the attic, roped to the wall and surrounded by spells preventing him from using magic.
- Eerie Pale-Skinned Brunette: What Ariel looks like, which is reflected in her Fetch's shadowy long black hair much like her own.
- Foster Kid: Ariel is raised by foster parents, having never met her real ones, and what little we see of her current family indicates she doesn't like them very much.
- Has Two Mommies: Charlie's fathers were briefly mentioned in The Witch Boy. Here, we meet them during the visits to her house.
- Heroic Sacrifice: When Ariel's Fetch grows too powerful and threatens to consume her, Mikasi absorbs it into himself just like the darkness he used to transform into a monster. Unfortunately, he lacks enough magic now to survive it, and the dark spirit kills him.
- I Just Want to Be Normal: Aster is not the only magical child apprehensive about their expected social role. Sedge no longer wants to be a shapeshifter after the trauma he received in the prior book. Being mutated into a monster against his will has made him fearful to try transforming again, and he develops interest in Charlie's descriptions of her public school. He uses his power one more time at the climax, but afterward still has no interest and decides to try living without magic.
- I Just Want to Have Friends: Ariel, due to her frequent isolation for being "weird". She very much appreciates Charlie's friendship, but has issues with being highly possessive of her, using her Fetch spirit to harm Charlie for not calling her phone after school, or attacking opposing basketball players during Charlie's game.
- Mental Fusion: What healing Mikasi entails, as a shared mind is needed to push out his corrupted magic. However, Grandma can't do it as Mikasi has too much anger toward her, so she wants Aster to try because he relates to Mikasi as a male witch. Aster initially horrified by the idea, and even after reneging finds peering into so much anger to be unsettling.
- She Is Not My Girlfriend: Ariel, jealous of Aster and Charlie hanging out, disdainfully refers to him as Charlie's "boyfriend". She denies any romantic interest in him, or really any romantic interest at all.