Many people think that characters in Keeper of the Lost Cities are LGBTQ+, which is normal, considering that there are many LGBTQ+ people in the world.
Many people think Dex is bisexual. This is shown through many instances and ships such as Detz/FedEx. Some people think that even though Dex doesn't hate Fitz anymore, he can't possibly like "Wonderboy" even if he stopped with the nickname, but others believe that the nickname is just a way of Dex trying to hide his feelings. He is also headcanoned as genderfluid, pansexual, nonbinary, and/or demiromantic.
Many people headcanon Biana as bisexual (Bi-ana), but others headcanon her as gay, dubbing her "Lesbiana" and pairing her with Sophie (Sophiana/Biophie), Marella (Marelliana/Biella), Linh (Linhiana/Binh), or both Linh and Sophie (Solinhiana).
Some people think the Mysterious Miss Foster is bisexual, asexual, pansexual, nonbinary, gray-ace, or lesbian, though it is not clear. She is shipped with Linh (Solinh/Lophie), Biana (Sophiana/Biophie), both Linh and Sophie (Solinhiana), and/or Marella (Sophella/Marphie).
Tam is headcanoned by many as gay, bisexual, nonbinary, transgender, aromantic and/or asexual, and shipped with Keefe (Teefe/Kam) due to them hating each other, similar to Drarry (Draco and Harry). He has also been headcanoned as trans.
Linh is mainly headcanoned as asexual and/or lesbian and liking Sophie (Solinh/Lophie), Biana (Linhiana/Binh), both Sophie and Biana (Solinhiana), or Marella (Marellinh/Linhella). She has also been headcanoned as trans.
Fitz is widely headcanoned as bisexual, transgender, gay, pansexual, genderfluid, and many others. People ship him with Dex (Detz/FedEx), again due to their supposed rivalry in the beginning of their friendship. People also ship him with Keefe (Feefe/Keefex) due to them being best friends.
Marella is headcanoned as being lesbian or bisexual and being romantically involved with Sophie (Sophella/Marphie), Biana (Marelliana/Biella), Stina (Stinella) or Linh (Marellinh/Linhella). She is also headcanoned as aromantic and/or demisexual.
 Alternating between sexualities, or not having fixed sexuality. Sometimes said to be "permanently questioning" or "sexualityfluid".
Attracted to males and masculinity from all genders.
 Identifying as having no gender.
 Not romantically attracted to others.
 Not sexually attracted to others.
Mainly attracted to one gender, but occasionally attracted to another gender. (See homoflexible and heteroflexible)
 Identifying as any combination of two genders.
 Attraction to two or more genders.
 Someone who identifies with the sex that they were born with.
Identifying partially as one gender (male, female, nonbinary, etc.), but partially as another (nonbinary, female, male, agender, etc.). This may be aligned with one's biological sex, or not.
Only attracted to those that they've formed a strong emotional connection with.
 Attraction to the same gender.(Lesbian refers to female homosexuals only, though Gay can refer to male and female as well as Homosexual)
Alternating between genders, or not having a fixed gender identity.
Only experiencing sexual attraction at certain times or in certain situations.
Only experiencing romantic attraction at certain times or in certain situations.
Attracted to females and femininity from all genders.
 Mainly attracted to the opposite gender, but occasionally attracted to other genders.
Mainly attracted to the same gender, but occasionally attracted to other genders.
 Stands for: Lesbian, Gay, Bisexual, Transgender, Queer/Questioning, and more.
Attraction to all genders.
A gender/gender spectrum outside of the male/female binary spectrum. Can also be used as an alternate term for genderqueer.
 Identifying as any combination of all/multiple genders.
 Attraction to all genders, or attraction to people regardless of gender.
 Attraction to multiple genders or all genders.
 Transgender means you were born in a body opposite to the gender you actually are. For example, you were born with the body of a girl, but you are a boy.
 Identifying as any combination of three genders.
 Someone who is unsure about their sexual/romantic orientation and/or gender identity.