Silas is Bod’s guardian, a vampire and member of the Honor Guard. When Silas goes away to fight the Jacks in San Francisco (without telling Bod why he’s leaving or where he’s going) and Bod misses Silas so bad, we’re told,
Silas had brought Bod food [...] but this was, as far as Bod was concerned, the least of the things Silas did for him. He gave advice, cool, sensible, and unfailingly correct; he knew more than the graveyard folk did, for his nightly excursions into the world outside meant he could describe a world that was current; he was unflappable and dependable and he had been there every night of Bod’s life. (3.34)
Without Silas, it would’ve been really hard, if not impossible, for the graveyard folks to raise Bod. As you can see from the quote it’s Silas’s wisdom that Bod values most about him. Silas has a presence that commands respect. He’s tough but fair, and a total sucker for Bod.
Miss Lupescu is what you might call a werewolf, and what she calls a Hound of God. She’s also a member of the Honor Guard, along with Silas. Silas arranges for her to teach Bod during most summers. Bod starts off not liking her at all, but then realizes that his survival depends on her teachings. Like Silas, Miss Lupescu is full of wisdom and she commands respect. Since she’s often in her human form, she can exist in the world outside the graveyard too, which makes her an excellent Silas substitute. Like Silas, Miss Lupescu would do anything in the world for Bod.