The complement in the sentence is friend (or the noun phrase "my best friend").
A complement can be a subject complement or an object complement.
A subject complement is a noun, a pronoun, a noun phrase, or an adjective that follows a linking verb and restates the subject.
A linking verb acts as an equal sign, the subject is or becomes the object (Jim = friend, or the noun phrase, my best friend).
- A subject complement that is a noun, a pronoun, or a noun phrase is called a predicate noun.
- A subject complement that is an adjective is called a predicate adjective.
The noun "friend" is the simple predicate nominative.
The noun phrase "my best friend" is the complete predicate nominative.
Example sentence for a predicate adjective:
My best friend is tall. (friend = tall)
An object complement is a noun, a pronoun, a noun phrase, or an adjective that follows a direct object restating the direct object. An object complement is separated from the direct object by a comma.
Example sentence for an object complement:
His brother Jim soon became my best friend, my buddy.
Note: In the sentence "His brother Jim...", the noun "Jim" is an appositive, a noun set beside another noun to explain or describe the noun. An appositive can be a noun, a pronoun, or a noun phrase and may be set off by a comma preceding and following the appositive.