On his first appearance, he already presents most of the characteristics he will have as a man. That is not to say that Tom does not change: he changes greatly as he matures. But the man is readily visible in the boy.
As a boy Tom is already strict with his sister, and fully convinced that it is for her own good. He is equally convinced that he can do no wrong. He sets his own standards of conduct, and so long as he maintains them he feels no pangs of conscience. And he always maintains his own standards. The fact that these often give pain to others — chiefly Maggie — is no concern of his.
His father's bankruptcy is the central event in Tom's life. Before it, he is a boy. After it, he is a man. The change is abrupt, but it is thoroughly convincing, for only a slight shift in values, and a slight increase in self-confidence, is involved. Tom the man is the same person. He is unimaginative but very clear-sighted. He always considers the possible, and will not look beyond that. Where, as a boy, he had little use for Maggie's imaginative games, now he easily gives up his own dreams of "cutting a fine figure." When the means are removed, the dream goes also.
In some ways Tom seems to have inherited the worst of both sides of his family. He has the Dodson strictness and respect for property, but not the strong feeling of kinship. He has his father's hard-headedness, his belief in himself, and his tendency to remember grudges; but he has none of his father's warmth and generosity. He cannot comprehend the nature of his father's charge to "care for the little wench." He sees "care" in terms of money and property, as would his mother's family, rather than thinking of love and kindness.
Only at the very end, when it is already too late, does Tom come to see that he has overlooked a large part of life. But that recognition brings him to a momentary reunion with Maggie before they die together.