Answer:
The relative pronoun in this sentence is 'that.'
Relative pronouns are used to introduce relative clauses. A relative clause is a clause that provides more information about a person, thing, or situation. The most common relative pronouns in the English language are: who, whom, which, that and whose. In the sentence: 'My mother is the person that has always believed in me', relative pronoun 'that' tells us more about the speaker's mother and the nature of their relationship.