someone called to a person's side to help, support, and speak for them before a higher authority
The word is built to mean "one called alongside." In the Greek and Roman world it named the influential person you brought with you when you had to face a court or a powerful patron: a defender who would stand at your side and speak for you. The same picture runs through Jewish tradition, where God's people imagined an advocate pleading their case in heaven against the one who accuses them. So the core idea is steady across every use: a powerful friend summoned to your side to take up your cause before someone greater.
The New Testament uses the word in two directions, but the underlying figure is one and the same. Jesus calls the Holy Spirit "another" helper of this kind (John 14:16), which assumes Jesus himself was the first. After Jesus leaves, the Spirit comes alongside the disciples to teach them, remind them of Jesus, witness to him, and stand with them in a hostile world. Meanwhile the risen Jesus himself keeps standing alongside his people in heaven, speaking for them before the Father (1 John 2:1).
No single English word holds the whole picture, which is why translators have never agreed. "Advocate" catches the legal defender; "Helper" and "Counselor" reach for the wider role of a supporter and guide; the old "Comforter" once meant "one who strengthens you by standing alongside," though that sense has faded. The word carries all of this at once: a defender, a witness, a guide, and a presence that stays with you.