A common noun can name a person, but a common noun does not name a specific person.
A common noun for a person is written in lower-case letters.
Here are some common nouns that name a person:
teacher
mother
father
neighbor
friend
doctor
nurse
lawyer
brother
sister
Any word that names a non-specific person is considered to be a common noun.
▶It’s your turn. Find two common nouns for a person in this sentence. Barbara is my friend, and her brother is Mark. Did you choose the two words for a person that are not capitalized? Good for you!