James "the Just" was the most important leader of the first Christian generation after the apostles, and the New Testament places him at the centre of the Jerusalem community. Paul, recounting his own early visits, calls him one of the "pillars" of the church alongside Peter and John, and names him plainly as "James the Lord's brother" (Galatians 1:19). In the book of Acts, it is James who gives the decisive judgment at the Jerusalem council on whether Gentile converts must keep the Mosaic law, a role that marks him as the settled head of the mother church.
His relationship to Jesus has been read in different ways. The Gospels list James first among the brothers of Jesus, and the plain reading of Paul and the Synoptics is that he was a sibling. Traditions that hold the perpetual virginity of Mary interpret "brother" as half-brother (a son of Joseph by an earlier marriage) or as cousin; the question turns on prior doctrine rather than on the texts alone.
What is not in doubt is his stature. From a family that, during Jesus' ministry, had stood at some distance from him, James emerged after the resurrection, to which Paul says the risen Christ appeared to him, as the authoritative figure of Jerusalem Christianity until his death.