Ah yes, I remembered incorrectly what Kanon was saying about the summoner's name. So it's a mundane western name that most likely belongs to some princess, and could possibly be shortened to "Beu" (though nicknames often arise form unexpected things). Right now I'm thinking of a Disney princess.
I did catch on to that David's sister has no birth record in her family, so she could be adopted. In that case, even if her her foster father had a religious naming convention, it might not apply to her.
Oh, and is "Joseph" really considered antiquated? I'm from a scandinavian country myself so I don't really know. I think it's probably a name of waning popularity, but I still considered Joseph pretty normal. But that would mean Jeroboam couldn't be David Joseph Martin, so I must've been wrong.
My initial hunch was that Walter is more involved in the story than he's letting in, because the only way he could have figured out the author is through the summoner's character and especially her name. I thought Walter might be someone related to the sister, though not Jeroboam (that'd be 2meta4me) But now that Renall's saying it's acceptable to treat Walter as his own man, I'm a little bit put off, but also somewhat relieved.