Ben Weatherstaff is a Yorkshire man and a crusty old gardener on the grounds of Misselthwaite Manor. When he first meets Mary, he tells her, "We're neither of us good lookin' an' we're both of us as sour as we look" (4.132). In other words, he sees that Mary is a brat but he admits that he is one, too. The one sign we get early on that Ben isn't totally crabby is his deep friendship with the robin: When he sees the robin coming by to visit, he tells Mary that the robin is "th' only friend [he's] got" (4.129). Is it a bird? You betcha. But it's also a friend.
Ben Weatherstaff doesn't become an important character until around Chapter 21, when he uses a ladder to climb up into the Secret Garden and he finds Mary, Dickon, and Colin there. It's then that we really confirm that Ben has a heart, if not of gold, then at least of some fairly valuable precious metal.
It turns out that, since Lilias's death, Ben has been climbing into the garden a few times a year to look after her roses in spite of Archibald's strict orders that the Secret Garden is supposed to be sealed off. Ben really liked Lilias, and he didn't want the roses she loved to die like that. He hasn't been able to keep up with the garden as of late because his bad joints make ladder-climbing a risky business, but he clearly feels protective of Lilias's garden.
Once Ben meets Colin and sees how much Colin resembles his mother, Ben joins Dickon, Colin, and Mary in their gatherings in the Secret Garden. Ben even starts to pray with the rest of them and to listen to Colin's theories about the Magic. Clearly, the Secret Garden is a special place if it can make even grouchy Ben Weatherstaff volunteer to hang out with other people.