Albert Barnes on Acts 21:18

Unto James. James the Less. Acts 15:13. He resided at Jerusalem. Comp. Gal 1:19. It is not improbable that he was the only one of the apostles then at Jerusalem; and there is reason to believe that the church at Jerusalem was left under his particular care. It was natural, therefore, that Paul and his travelling companions should take an early opportunity to see him. James was the cousin of our Lord; and, in Gal 1:19, he is called the Lord's brother. On all accounts, therefore, he was entitled to, and would receive, particular respect from the early disciples.

(d) "James" Acts 15:13, Gal 1:19