James Henry Alden is Henry James Alden, Jessie Alden, Violet Alden, and Benjamin Alden's kind and wealthy paternal grandfather, whom they live with in Connecticut. In the first book after the children's mother and father die in a car accident, the children run away, afraid that he might not like them and that he was mean because he did not like their mother, his daughter in-law. The prequel novel gives his son's name as Ben.
He has one sister who is about a year older than he is, Aunt Jane, and a brother who was several years (15-20 years) younger than he is (unnamed). This younger brother is Joe Alden's father. James is said to own several mills (paper, plastic, etc), refineries, a woodlot, a boat, a private island, a private plane, a farmhouse outside of Boston, a ranch house in Pennsylvania, and is the main patron of the town museum.
It is hinted that his favorite color is yellow.
It is implied that he is almost 70 years old in book 3.
The new animated movies clarify that he is a steel baron, and that this is the source of his wealth.