In spoken English and in some informal writing, whom is rarely ever used, but in formal writing, you must determine whether to use who (nominative) or whom (objective) when introducing a relative clause. To help you recognize whether the pronoun is being used as a subject (nominative) or an object (objective) within the relative clause, try rephrasing the clause as a sentence and substituting a different pronoun for who/whom.
How do I decide between who and whom?
I met the artist [who/whom painted this picture].
Rephrased: She painted this picture.
Correct: I met the artist [who painted this picture].
In the above relative clause, she is the subject of the verb painted; therefore the nominative case is required.
He is the kind of person [who/whom I admire].
Rephrased: I admire him.
Correct: He is the kind of person [whom I admire].
In this second example, him is the object of the verb admire; therefore the objective case is required.
Remember that the key question is how the pronoun functions with the clause.