Why obsess over Walter? Because he's amazingly lovable.
Walter's appreciation in Hellsing fandom has been picking up recently, as he's come out of the closet woodwork. Early in the story he does a lot of melting into the background.
But that's part of Walter's charm, you see: he's so good at being a nonpresence. He's the perfect quiet and mild-mannered butler, and yet he helps Hellsing run smoothly, from making sure Integral gets her mail to designing weapons for Alucard and Seras. He's almost easy to overlook, but if you think of the Hellsing organization as a well-oiled machine, he's the one with the oilcan.
And if you cause trouble, he can go to the bathroom cupboard, pull out the dental floss, and slice you into little tiny pieces.
Okay, it's actually monofilament steel, but it's basically floss without the wax or minty fresh flavor. It slices. It dices. It cuts through ghouls, tools, and fools. It's the most original weapon in the whole series, and one of the best in anime/manga in general. And he's been wielding the wires with expert deftness since age fourteen, and can still do so at your grandparents' age.
When the floss isn't out, though, Walter's irresistably likeable for the sake of his personality. Except after catastrophes, Walter's always smiling. Alucard has an omnipresent smirk, Seras occasionally has a nervous grin, and Integral hardly ever lets her lips so much as twitch upwards; but Walter is a generally happy and relaxed man, and it's apparent in his smile.
This attitude puts Walter on good terms with just about everyone in Hellsing. He's devoted to Integral, and is one of the few people she trusts; he can cheer her up in the most dire of situations. He's on a first-name basis with Alucard, and can converse with the formidable No Life King comfortably and naturally. And, unlike both of those two, Walter doesn't scare Seras. In fact, he's openly friendly to her. He should have a place in the heart of anybody who's a fan of one of those three. He's got one in mine.