This is the 1969th most frequent Spanish word.
tú
"You" (informal singular).
In this sentence, 'tú' is used as the subject pronoun to emphasize that the person being spoken to is the one being asked about their interest in going to the cinema.
¿Tú quieres ir al cine esta noche?
Do you want to go to the cinema tonight?
Here, 'tú' serves as the subject pronoun within the subordinate clause 'tú eras especial para mí,' identifying who is being referred to as special.
Te dije que tú eras especial para mí.
I told you that you were special to me.
In this case, 'tú' emphasizes the subject, specifically clarifying that the person being addressed is the one who potentially left the letter.