In this sentence, "someone like you who is..." , 'someone' is the antecedent of relative pronoun, 'who' .
But can you explain why 'you' cannot be the antecedent of the relative pronoun, 'who' ?
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In theory, an ambiguity could arise.
We could write a sentence such as "I am looking for someone, like David" (i.e. like David is) or "I am looking for someone, like David, who has been looking for a long time now".
If the noun or pronoun after "like" is the subject of what follows, there should be a comma before "like" (as in my examples), but not everyone is fastidious about comma use.
In your expression "someone like you who is", it could be argued that, if "you" were the subject, the verb should be "are". But this rule isn't always followed after "who" in informal English. Many speakers would probably find "It's you who's responsible" just as acceptable as "It's you who are responsible".
Still, in an expression such as "someone like X who is ...", there is a strong presumption in favour of "someone" being the antecedent of "who" - unless there is a comma to suggest otherwise.