Final answer:
The subject pronoun to address a good friend in Spanish is 'tú'.
Step-by-step explanation:
The subject pronoun that you would use to address a good friend in Spanish is tú. It is the informal singular pronoun and is used when speaking to friends, peers, or people of the same age group. For example, in the given sentence 'No eres agresiva. Eres tranquila.', the subject pronoun 'eres' is used to refer to a good friend.
Learn more about Subject pronouns in Spanish