A friend recently texted me 'いつ帰ってくるんー？'. I understand the question, but I don't understand why んー would be added to the end of the verb. Is this to add emphasis or emotional context?
「いつ帰{かえ}ってくるんー？」
「ん」＝「の」
Around Kansai, it is common to end a question with a 「ん」 in colloquial speech. The 「ー」 should be for emphasis as you said.
Thus, a more "Standard" way to say the same thing would be:
「いつ帰ってくるの～？」
Kansai ladies have asked me questions such as:
「うちのこと、ホンマに好{す}きなん？」 "Do you really like me?"
「最近{さいきん}めっちゃ回答{かいとう}したはるけど、ヒマなん？」 "You've been answering a lotta questions lately. Are you bored silly?"
Disclaimer: If your friend is not from around Kansai, please ignore this answer. I would have no idea what 「んー」 meant, then, in his/her dialect. It is always helpful to tell us where the speaker is from when asking these questions.
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1Note that Tokyo people sometimes use it as a slang too. Some as faux-Kansai style and others don't even seem knowing it is. – broccoli facemask - cloth Aug 1 '19 at 7:41
