What kind of complement is the word unique

Unique means to be unlike anything else. When something or someone unique, they are one of a kind. It is often a very nice compliment when meant as a compliment. Sometimes unique can be used as a placeholder for a compliment when someone does not wish to insult the other person. If you think something is ugly or bad, you can say unique, because the thing or person can still be unlike anything else, even if it's not good.

There are five sentence complements. Three are Action Verb Complements and two are subject complements

Action Verb Complements
1) Direct Object - a noun or pronoun that receives the action of the verb
2) Indirect Object - instances when direct object also has indirect object; it is affected by the action given to the direct object
3) Object Complement - modifies or renames an object

Subject Complement (used only with Linking Verbs)
4) Predicate Nominative - renames the subject of the linking verb
5) Predicate Adjective - modifies the subject of a linking verb.

For example: She is unique. - unique is a predicate adjective for it modifies the subject of the linking verb.