When the relative pronouns who, which, and that are used as a subject, the verb agrees with the antecedent of the pronoun.

Problems often occur with the words one of the. In general, one of the takes a plural verb, while only one of the takes a singular verb.

Some employees always work overtime. Carla is among them. Thus who refers to employees, and the verb is plural.

Only one employee always works overtime, and that employee is Ming. Thus one, and not employees, is the antecedent of who, and the verb form is singular.