Non-binary (NB) is a catch-all category for characters whose gender identities that are not exclusively masculine or feminine. Instead, their identities are outside of the gender binary and cisnormativity.
Non-binary people may also identify as transgender and/or transsexual. The label genderqueer has a lot of overlap with non-binary.
Non-binary people may wish to transition so that their gender expression more closely reflects their internal identity. Many non-binary people wish to appear androgynous and adopt unisex names, gender-neutral titles such as Mx. and/or gender-neutral pronouns, but others prefer to express themselves in ways which are traditionally seen as masculine or feminine or to mix aspects of the two.
Non-binary people can have any sexual orientation, although if attracted primarily to a single gender they may prefer to use gender-terminology to express this, such as androsexual or gynosexual.