The same for Portuguese. You can omit it. But if you want to contrast people, highlight the person, or even not to get the subject misunderstood, we normally use the pronoun.
No, "amargo" shows how the taste is like. Using DE shows what the coffee is made of.... like "café de chocolate" (made of chocolate) I know it does not exist, but just to show how you would make a sentence. Amargo here is jist an adjective...