They

Here's a comprehensive summary of the Wikipedia article about the pronoun "they":

The Modern English pronoun "they" has five distinct forms, including the nominative "they," objective "them," and possessives "their" and "theirs." Imported from Scandinavian languages like Old Norse around the 13th century, it gradually replaced Old English third-person plural pronouns, with its usage solidifying by Chaucer's time. A significant feature is "singular they," an epicene (gender-neutral) pronoun for a single referent, which, despite its singular meaning, follows plural grammatical agreement. This historical usage for individuals, whether of unknown gender or as a non-binary pronoun, spans over 600 years and led to Merriam-Webster naming singular "they" their Word of the Year in 2019. Beyond referring to a group, "they" functions variously as a subject, object, or determiner, with its singular form exclusively referring to people.