which kind of clause is I have known since kindergarten is this in the sentence: Wendy is someone I have known since kindergarten.

This clause is a noun clause because Wendy is a noun.

Explanation:

A noun clause is a needy condition that goes about as a thing. Thing provisions start with words, for example, how, that, what, whatever, when, where, regardless of whether, which, whichever, who, whoever, whom, whomever, and why.

Noun clauses can go about as subjects, direct items, circuitous articles, predicate nominatives, or objects of a relational word.