Precocious. Mature. Way beyond his eleven years. Eliot's super-observant and perceptive (which is why the story works from his perspective).
He notices little things, like how his mother doesn't really eat the food Mrs. Sen offers her (MS 29-31). He can tell when Mrs. Sen feels sad (usually when she's thinking about "home," i.e., India) (MS 18-22).
He's also lonely—that is, before he meets Mrs. Sen. Once he goes to Mrs. Sen's, the beach where he lives with his mother becomes "barren and dull to play on alone; the only neighbors who stayed on past Labor Day, a young married couple, had no children, and Eliot no longer found it interesting to gather broken mussel shells in his bucket, or to stroke the seaweed" (MS 12).
Mrs. Sen gets Eliot to talk about what he really thinks. For example, when she asks him if he would ever put his mom in a nursing home, he says "Maybe…but I would visit everyday" (MS 107). How's that for honesty?
Eliot thrives on the attention he gets from Mrs. Sen. Which is why, even though he doesn't make a fuss about it at all (Eliot's all about being "no trouble" (MS 30)), we know that he really misses Mrs. Sen once his mother pulls him from her care.
How do we know this? On the first day that he's left at home alone, his mother calls and asks him how he's doing. This is how his story ends: "Eliot looked out the kitchen window, at gray waves receding from the shore, and said that he was fine" (MS 129).
An 11-year-old, sitting and staring at gray waves? That's a lonely, desolate picture.