Sibyl's large and brash brother is very protective of her. He vows to kill anyone who harms Sibyl.
Distrustful of his mother’s motives, he believes that Mrs.Vane’s interest in Dorian’s wealth disables her from properly protecting Sibyl. As a result, James is hesitant to leave his sister. He moves to England to become a sailor. He is suspicious, and even don’t like his sister’s new love, and he swears he will kill everyone who will try to hurt her. Looks like he loved the sister very much, being an extremely caring brother. After he finds out that the sisters commit suicide, he dedicates his life in order to find the person who is responsible – “Prince Charming.” In a struggle to find and killed the “Prince,” he got killed him by the wayward hunting bullet.
He's kind of a super-masculine man in a world full of more ambiguous males; a sailor, traveler, and all around tough guy, he seems to represent a world of brawn over brains that Wilde's not terribly interested in.
James Vane is a simple character—he's totally motivated by love, jealousy, and revenge. He adores his sister, Sibyl, and we can see his promises in the citations below:
"He is gone," murmured Sibyl sadly. "I wish you had seen him."
"I wish I had, for as sure as there is a God in heaven, if he ever does you any wrong, I shall kill him."
She looked at him in horror. He repeated his words. They cut the air like a dagger.
That being said, he's not actually a bad guy—after all, he feels truly awful when he thinks he almost killed the wrong man. Really, he's just one of fate's punching bags—James isn't clever or handsome enough to make it in Wilde's world.
James Vane in the Essays