Dickon Sowerby, Martha's twelve-year-old brother, is basically a mensch. There's really no other word for him: He's a stand-up guy who rescues orphaned foxes and crows from drowning on the Yorkshire moors and raises them up to be his own. He takes care of a kitchen garden to help his mother feed his many brothers and sisters, and, of course, he also steps in to help train Mary and Colin into great gardeners. He's an all-around great person who loves his homeland on the moors and seems to have a natural gift for looking after all living things—be they people, plants, or animals.
Dickon may not be as central a figure in The Secret Garden as either Mary or Colin, but he's at the novel's emotional heart. When Martha first mentions Dickon to Mary, she tells Mary that "th' very birds likes him an' th' rabbits an' wild sheep an' ponies, an' th' foxes themselves" (7.22). If birds and rabbits and foxes all like Dickon, then surely we have to like him, too.
Not only is Dickon gentle and kind to the living things around him, but we also get plenty of insight into the quiet, uncomplaining work that he puts in helping out his family. When he's not working with Mary and Colin on the Secret Garden, he's out in his own garden out on the moor "planting or tending potatoes and cabbages, turnips and carrots and herbs for his mother" (24.1). Gardening turns out to be essential to Mary and Colin's health and happiness, but it's life and death for Dickon's family—without the veggies he grows, the Sowerbys "would never get on as comfortable as [they] do" (24.2).
While Dickon has pretty much all of the positive traits the novel could give him, he doesn't get much of a character arc. He starts out great and he ends great—which makes him fairly two-dimensional as a character. Still, without him, neither Mary nor Colin would be able to improve their health and happiness as much as they do. Even though he isn't much older than either of them, he still acts as a kind of mentor in their search for a better life. After all, he has what they lack: a close connection to nature.
Dickon obviously has an amazing instinct for the workings of the natural world. And while the novel doesn't often put this natural instinct into religious terms, the connection between Dickon's love of nature and a love of God does appear here and there in the later chapters of the book.
In the second-to-last chapter, when Colin feels particularly full of Magic in the Secret Garden, Dickon leads him, Mary, and Ben Weatherstaff in the Doxology, a song of praise to God. Hearing Dickon sing makes even Ben Weatherstaff admit, "I never seed no sense in th' Doxology afore […] but I may change my mind i' time" (26.37). In other words, while Dickon isn't trying to convert anyone to Christianity, the strength of his own faith and his own convictions affect everyone around him.
The Secret Garden definitely doesn't have a rigid, strictly Christian message—Colin talks more about Magic than about more traditional words for God or faith—but there is an undercurrent of connection in The Secret Garden between a love of nature and a love of God. And Dickon is the character in whom this faith in both forces gets its clearest expression. If you want to compare Frances Hodgson Burnett's real-life beliefs with what you see in this novel, go read our "Trivia" section for some notes on Frances Hodgson Burnett's interest in Christian Science and mysticism.