Walter's a man of many faces. Or a man who wears many hats. Take your pick of metaphors. He does a lot.
The face (or hat) that we're first introduced to is the "butler" persona. Walter greets Seras and shows her to her room. He delivers her food. He makes tea. He hands Integral her cigars. This is all in accord with the first impression he makes.
But Walter also wears the hat of weapons technician; he makes Alucard and Seras new guns. And then we got to see a glimpse of an unexpected and pleasantly surprising face: the sci-fi geek. Along with reading classical literature, Walter's a Dune fan.
Then comes the face that knocks people out of their socks the first time they see it: that of Shinigami Walter (literally "god of death", but the Japanese concept translates better into the Western "Angel of Death"). This hat's been on the shelf for a while now, but sometimes Walter needs to put it back on. Along with a special set of gloves, which come with those infamous wires.
"Angel of Death" is Walter's alias from back in the days of WWII, when he had a very different face - a much younger one. Though old for his age (the military will do that to you), he was still a teenager in some ways. He's gotten more Zen over the years.
For those fans of the younger version, though, you're in luck. In the latest issues of Hellsing, Walter's been captured - and turned into a thirtysomething version of himself, addressed as Butler. His new face is wrinkle-free, has hair falling in it, and has red eyes. It also doesn't smile a lot.
Walter has worn the hats of caretaker, confidante, and friend. Whether he'll ever put any of them back on remains to be seen.