Katrina Merton
Portrayed By: Katharine Isabelle (TV)
The protagonist. She finds the Grool under the sink of her new house, and she wants to get rid of it.
- Adaptation Dye-Job: Is said to have "reddish" hair in the book, but has brown hair in the TV episode.
- A Birthday, Not a Break: She wants to go to a water park with her friends for her 12th birthday. However, a rainstorm on that day has her mother cancel the trip. This makes Katrina furious at the Grool, believing its bad luck is to blame.
- Color Motif: She likes to dress in outfits that include a range of pink and purple clothing, her favorite colors.
- Crying Wolf: She is smart enough to realize her parents won't believe her if she tells them about the Grool and its powers, but she tries it on her aunt and her teacher. She is not believed by either of them.
- Daddy's Girl: She mentions how she is very like her father. They have the same favorite foods, favorite programs, and favorite activities.
- Determinator: When she finds out that she lost the Grool, and realizes that anyone who loses the Grool for 24 hours DIES, she is adamant to get this creature back no matter what.
- Earn Your Happy Ending: She defeats the Grool, there is no more bad luck in the Merton household, and she is safe. But a certain Lanx could cut the happy ending short.
- Embarrassing Nickname: Her father likes to call her "Kitty-Kat". Katrina does not appreciate this nickname.
- Fiery Redhead: Has red hair, and a bit of a temper, especially when it comes to dealing with Daniel and Carlo's antics.
- Foolish Sibling, Responsible Sibling: The Responsible to Daniel's Foolish.
- Genre Savvy: She realizes that if she tells her parents that the "sponge", the one that she and Daniel had been fighting over, is the one causing bad luck, her parents won't believe her and think that she's just lying about it to get away with misdeeds that had been happening recently.
- Girly Girl with a Tomboy Streak: She is shown to be fairly girly, with a fascination for fashionable clothing and colors. However, she's not above certain hijinks with Daniel, and doesn't particularly mind going down a slimy sewer to find the Grool.
- Nerves of Steel: She went up against a few high schoolers who had taken the Grool and she wanted it back. It's even mentioned that they looked big enough to beat her up, but she tries it anyway.
- Only Sane Man: Feels this way when being around Daniel and Carlo.
- Sibling Rivalry: Has this with Daniel, even going so far as fighting with him over what they first believe is an old sponge, which their mother exasperatedly lampshades. Then again, Katrina DID realize that this was no ordinary sponge, and was challenging Daniel.
- Uncertain Doom: She ends up holding the cousin of the Grool, which is the Lanx, a creature that can clamp down on a victim and only let go when he or she has completely drained away energy. It is unknown if the Lanx did this to Katrina. However, Goosebumps Wiki lists her down as "Alive", and she's smart enough to throw it off her, so she probably survived.
- Youthful Freckles: She mentions that she has many freckles on her face, and she turns 12 halfway through the book.
The Grool
An evil sponge-looking creature that bestows bad luck on anyone who comes into possession of it - and death to anyone who tries to get rid of it.
- Abstract Eater: It feeds off bad luck.
- Big Bad: Of It Came From Beneath The Sink!
- Clingy MacGuffin: Once you discover one, you're pretty much stuck with it for life. Attempting to get rid of it will only cause you to die after you are separated from it for too long. The only known way to get rid of it is being kind towards it.
- Eldritch Abomination: It's essentially a sponge with glowing eyes - and in the TV episode, it has teeth as well.
- For the Evulz: It literally feeds off of bad luck and negative energy, and gets stronger and healthier the more people around it suffer.
- Reverse Psychology: The protagonists manage to defeat it by literally killing it with kindness.
- Spared by the Adaptation: In the book, the protagonists managed to kill it by realizing kindness weakens it. In the series, it's not enough to actually kill it, so they instead trap it inside a small prison of various nice things to prevent it from growing again.
