Excellinor the Witch

When Alvin tricks Hiccup in How to Break A Dragon's Heart, Hiccup thinks he is being led to Camicazi's prison, but he is actually led to Alvin's mother's prison, and left inside. Alvin's mother is, supposedly, a witch.

When Hiccup escapes from the prison, he forgets to lock the door properly, allowing Alvin's mother to escape. Later, Alvin's mother is falling off a bridge, and tells Alvin to get the sword. Alvin, thinking she means the Stormblade, tells her he has it, but she says no, the other one. She meant Endeavour, the sword Hiccup was using.

How to Break A Dragon's Heart is the only book Alvin's mother is in.