What should I
call them in Korean?

Korean titles depend on relationships.

Words such as Oppa, Unnie, Hyung, Noona, Sunbae, -ssi, and -nim often reflect age, gender, social position, and closeness. They cannot always be translated directly into English.

This guide offers a natural starting point, not a rule to impose. In real life, follow the relationship, the setting, and how the other person prefers to be addressed.