Protagonists

The Main Trio

Beauty, Brains, and Brawn: Emma fits the role of the more caring and gentle Beauty, Pao fits the role of the intelligent Brains, and Dante fits the role of the more hands-on heroic Brawn.
Child Hood Friend: All of them are this to each other.
Freudian Trio: Dante is the Id, who mostly makes his decisions made on instincts and emotions, Pao is the Superego, being more focused on ration and logic, and Emma is the Ego — as Pao puts it, Emma was the stabilizer of the group, and the one who balanced it out.
Two Girls and a Guy: Emma and Pao are both girls, and Dante is a guy who has the starts of a romantic relationship with Pao.

Paola "Pao" Santiago

Disappeared Dad: Pao has never known her father. She's not really bothered by it for the most part.
Locked into Strangeness: One of Pao's braids and one of her eyebrows are bleached white after using the key.
The Smart Guy: Pao is extremely intelligent and often thinks about a wide range of scientific matters, from algae to space exploration.
Wacky Parent, Serious Child: Pao is often embarrassed of her mother, who believes in magic and ghosts and often lets her clients get away with not paying her with money despite the fact they're really poor, and Pao fancies herself a scientist and only believes in logical things that science can explain.

Dante Mata

Chronic Hero Syndrome: This is mentioned a lot — Dante has a bit of a savior complex where he automatically wants to save those he loves when they're threatened, which is the thing that actives his club. Pao finds this somewhat annoying at first.
Lovable Jock: Downplayed — Dante is on the soccer team, but he's a nice, heroic person.
Morph Weapon: The chancla Senora Mata gave him can turn into a powerful club.

Emma Lockwood

Les Yay: It's implied Emma had a crush on Ondina, which is why she went with her to the river, when Pao asks why she would try to impress someone she just met and Emma blushes.

Naomi

Marisa

Reformed Bully: Marisa was previously a bully of Pao's, but she matures after her presumed death and apologizes when she's informed about that past.

Sal

Children Are Innocent: Sal, despite having gone through a lot, believes in Pao despite what everyone else says, and is the first to tell her the truth.

Bruto

Cute Monster: He's a chupacabra puppy who quickly bonds with Pao.
Loyal Animal Companion: Bruto, despite only knowing Pao for less than a day, still follows her into the Rift, comforts her when she's upset or scared, and defends her from other monsters.
The Confidant: Pao shows a softer side to Bruto than to almost anyone else, since he's not actually a person and only offers unconditional love and support.

Ondina

Enigmatic Minion: For a good portion of the book, Pao is unsure of whether Ondina is to be trusted, even though she has some measure of control over the monsters. She also doesn't know the truth about Ondina and La Llorona until Ondina tells her.
Heel–Face Turn: Ondina undergoes one of these after talking with Pao, although she's still a bit prickly and snarky.
Morality Chain: Ondina serves as this for La Llorona, being the one who convinces her to finally let go.