The use of 'they' is just because Japanese doesn't use gendered pronouns as much as English does, which complicates translation sometimes. It's not like using 'they' is always plural; it's been used as a single-person genderless pronoun in English for centuries.
"There's one high-ranking adventurer who willingly gets soaked in blood. Their name is Luka," and they live on the second floor. She lives upstairs from you. Yes, I think you've seen her before.