IPA: /ˈkaʊn.sə.lər/
KK: /ˈkaʊnslər/
A person who provides guidance or advice, often in a professional setting, such as a therapist or legal advisor.
She decided to see a counselor to help her with her personal issues.
Counselor is formed from "counsel" (from Old French *conseil*, meaning advice or consultation) and "-or" (meaning a person who). A counselor is a person who provides advice or guidance.
Think of someone giving 'advice' ('counsel') — the '-or' indicates that this person is the one providing that advice.