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 and demiromantic.
Many people headcanon Biana as bisexual (Bi-ana), but others headcanon her as gay, dubbing her "Lesbiana" and pairing her with Sophie (Sophiana), Marella (Marelliana), Linh (Linhiana), or both Linh and Sophie (Solinhiana).
Some people think Mysterious Miss Foster is bisexual, asexual, nonbinary, gray-ace, or lesbian, though it is not clear. She is shipped with Linh (Solinh), Biana (Sophiana), both Linh and Sophie (Solinhiana), and/or Marella (Sophella).
Fitz is widely headcanoned as bisexual, transgender, gay, pansexual, genderfluid, and many others. People ship him with Dex (Detz or FedEx), again due to their supposed rivalry in the beginning of their friendship. People also ship him with Keefe (Feefe) due to them being best friends.
Wylie is headcanoned as gay and/or a trans boy (born in a female body but identifies as a male).
Marella is headcanoned as being lesbian or bisexual and being romantically involved with Sophie (Sophella), Biana (Marelliana), or Linh (Marellinh). She is also headcanoned as aromantic and/or demisexual.
Identifying as having no gender.
Someone who is not romantically attracted to others.
Someone who is not sexually attracted to others.
Mainly attracted to one gender, but occasionally attracted to another gender. (See Homoflexible and Heteroflexible)
Identifying as two genders at once.
Someone who is attracted to two or more genders.
Someone who identifies with the sex that they were born with.
Someone who is only attracted to those that they've formed a strong emotional connection with.
Someone who is attracted to the same sex. (Lesbian refers to female homosexuals only.)
A person who alternates between gender identities, or does not have a fixed gender identity.
Someone who only experiences sexual attraction at certain times or in certain situations.
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.
Attracted to all genders.
Identifying as a gender outside of male or female. Can also be used as an alternate term for genderq***r.
Identifying as multiple genders or all genders.
Attraction to all genders, or attraction to people regardless of gender. 
Attracted 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.
Someone who is unsure about their sexual orientation and/or gender identity.