So, tú is an object pronoun in this sentence, but Duolingo isn't teaching us to use "thú" here. Is this similar to who/whom in English, where there is the grammatically correct way of saying something, and then there is the often done and socially acceptable way of doing so? (For example, "whom do you like?" is more commonly phrased, "who do you like?")
It's not a proper English sentence. It would sound like you are saying "I like you! Really!" which is something you would say to someone when you are trying to convince then that you don't dislike them. "I really like you" means that your affection towards them is greater than they might realize.