What we are really saying here is ぼくのうしろにいる のは だれ the は is omitted colloquially often by children or people who have a close relationship with each other. Other comments are saying this の has to do with location and that is incorrect.
Other ways this ‘no’ may be used
まゆこにラインしたの何時？ What time did you text Mayuko?
たくやが買ってきてくれたのどれ？ Which one did you/Takuya bring?
These are all actually のは technically, just omitted.の is often used in place of こと which is used in place of specific nouns that you already know from the context of the phrase.
So what is really being said is:
僕の後ろにいる人はだれ？but we already know it means ‘hito’ person because the question is who.
ラインした時間は何時？but we already know it is ‘jikan’ time because it is when.
買ってきてくれたプレゼントはどれ？we don’t know just from this sentence what the actual thing is, but it would be used in a setting where the speaker is asking the listener in front of whatever it is they brought, so we know it is ‘mono’ or thing (in this case I just threw in present) because the question is which.
Because all this information can be inferred from context we omit and shorten by just saying の.
3 comments
You’re correct. の here functions by turning “ぼくのうしろにいる” into a noun phrase.
Check the replies here
https://japanese.stackexchange.com/questions/33027/about-the-second-%e3%81%ae-in-%e4%b8%8d%e6%80%9d%e8%ad%b0%e3%81%ae%e5%9b%bd%e3%81%ae%e3%82%a2%e3%83%aa%e3%82%b9
What we are really saying here is ぼくのうしろにいる のは だれ the は is omitted colloquially often by children or people who have a close relationship with each other. Other comments are saying this の has to do with location and that is incorrect.
Other ways this ‘no’ may be used
まゆこにラインしたの何時？
What time did you text Mayuko?
たくやが買ってきてくれたのどれ？
Which one did you/Takuya bring?
These are all actually のは technically, just omitted.の is often used in place of こと which is used in place of specific nouns that you already know from the context of the phrase.
So what is really being said is:
僕の後ろにいる人はだれ？but we already know it means ‘hito’ person because the question is who.
ラインした時間は何時？but we already know it is ‘jikan’ time because it is when.
買ってきてくれたプレゼントはどれ？we don’t know just from this sentence what the actual thing is, but it would be used in a setting where the speaker is asking the listener in front of whatever it is they brought, so we know it is ‘mono’ or thing (in this case I just threw in present) because the question is which.
Because all this information can be inferred from context we omit and shorten by just saying の.