Brother of Jesus, leader of the Jerusalem church. He did not believe in Jesus during the ministry (John 7:5). He was converted by a post-resurrection appearance (1 Cor 15:7). He then became the pillar of the Jerusalem church — the Jewish Christian center Paul sometimes had to negotiate with. His letter is practical wisdom, Torah-shaped. He was killed by the Sanhedrin in AD 62.
On their voice
Practical, Torah-grounded, the voice of Jewish Christianity that predates Paul's theology and does not share all of it. Not a villain — a different emphasis, rooted in land and law and community.
Talk to James of Jerusalem.
Ask anything. In their own voice, from their own era, grounded in their own canon.