Keckwick is the driver of the pony and trap (kind of like an old-school cab driver) who delivers Arthur to and from Eel Marsh House, since no one else will set foot near that place. At first glance, you might not consider Keckwick the most comforting of characters. First of all, he hardly says a word:
For answer, he simply pulled on the pony's rein […]. Minutes later, they were receding across the causeway. (5.15)
However, he represents Arthur's one connection to town and thus to reality while he's stuck at Eel Marsh House. Arthur relies on Keckwick to come back and get him when no one else in the town seems willing to come within spitting distance of Eel Marsh House. And Keckwick looks out for Arthur, just like Sam:
"I wouldn't have left you over the night," he said at last, "wouldn't have done that to you." (6.28)
Keckwick may not be chatty or fun to be around, but he's dependable, and that's exactly what Arthur needs.