A katakana rendering of the second-person pronoun 'you'. In Japanese it sounds foreign, playful, or affected, so ordinary speech more often uses a name, title, or pronouns such as あなた, 君, or お前 depending on the relationship.
ユー、次は何を歌うの？
You, what are you going to sing next?
彼
He jokingly called me 'you.'
Written in katakana and used to represent the second-person pronoun 'you' in Japanese text. No language-of-origin information is specified, so the historical borrowing details are not specified here.