Haley got Quippie in bonus strip in Utterly Dwarfed (it did not appear on the web). On the other hand, I have no idea who the Senri's troll is.
It's 42 strips from the beginning so far, halftime for BRitF was about 138 strips, and this last book will likely be even longer. Plenty of time yet. I also thought Quippie may be it, but now I'm convinced it was introduced as a red herring precisely for these speculationsWe're a decent bit into the first half of the seventh book by now, and still no obvious candidate for the Major Character has appeared. I'm starting to worry that something underwhelming will wind up being the case, like Quippie being the "Important Ally" just because his corpse helped the Order bypass Serini's security.