Learn Portuguese (Portugal): Quem é o amigo?

Just like in English, questions starting with a question word like quem (who) put that word right at the beginning. The verb follows immediately after: Quem (Who) + é (is) + o amigo (the friend).

Why do we need the word o here? Can I just say Quem é amigo?

In Portuguese, you almost always need a definite article like o (the) before a noun when referring to a specific person. If you drop it and say Quem é amigo?, it sounds more like a general philosophical question meaning "Who is a friend?" or "Who is friendly?".

Why do we use é and not está for "is"?

The word é comes from the verb ser (to be). We use ser to talk about permanent traits, including a person's core identity or who they are. We only use estar for temporary states, feelings, or locations.

How would I ask this if the friend is a woman?

You would need to change both the definite article and the noun to their feminine forms. The question would become Quem é a amiga?