@Crossword:
Jeremiah: Switched sides way too quickly. How'd he go from research head to…of yeah, he was supposed to die after Narita. Those fans, maybe they'll make a way to negate what happened next...
One of the Geass themes is loyalty to one's lord, it seems to be a common trait in the best knights of the Britannian Empire. Guilford, for example, would probably be willing to betray Britannia on behalf of Cornelia should she go there.
As it turns out, Jeremiah was once Marianne's bodyguard and, like almost everyone who knew "Marianne the Flash" in person he was totally dedicated to her. He failed to save her, then he heard of her children Lelouch/Nunnally get killed in the Battle of Japan, so that was like two failures for him. So he transferred there and joined the Purist Faction (blaming the Japanese for the alleged killing) and was screwed over a third time by Zero, ironically Lelouch in disguise.
Of course all this was detailed in the drama CDs so nobody would have seen it coming if they didn't get the additional info! Such is a weakness of the show's writing, to leave everything important out of it so there can be ridiculous plot twists in every episode.
I'm rather happy Jeremiah joined Lelouch…not only because it was Jeremiah, but finally some of Marianne's vanguard and other Britannians will assist Lelouch in his conquest. Up until this point in the story, Zero's identity as a Britannian Prince has been nothing but a liability, but Lelouch's past can command some powerful allies now toward the conclusion of the series who are not just dedicated to "Zero" but to Lelouch's actual goals.
Jeremiah is the first Black Knight (if we can call him that right now) aside from C.C. who is battling for the goals of a) destruction of Britannia in revenge for the murder of Marianne and b) create a better world from the rubble where the weak won't be oppressed. Everyone else is fighting for Japanese nationalism, allying with Zero because they know he's somewhat interested in them and can produce results. A different brand of loyalty.
If events pass as some believe they will (the Black Knights oust Zero when Japan is conquered because they don't want to further aggravate Britannia) Lelouch is going to need powerful allies like Jeremiah to tie him over until the Black Knights change their minds when there's another invasion.