Αγαπώ τον Λουί. (Agapō ton Louī.)
* Αγαπώ (Agapō): I love (verb, present tense, 1st person singular)
* τον (ton): the (masculine accusative article, necessary before a proper noun)
* Λουί (Louī): Luis (the name)
You could also use Σε αγαπώ, Λουί (Se agapō, Louī) which translates more closely to "I love you, Luis," but is still perfectly acceptable and often used. The difference is subtle and depends on context.