Second in command to Chief Lector Iskandar, Desjardins is a stereotypical stuck-up French guy—except he's also a powerful magician. Distinctive with his cream-colored robes and his forked beard (seriously, did he step out of the Three Musketeers or something?), Desjardins is one of the many magicians who want to uphold the anti-god laws no matter what.
One of the reasons for his loyalty to the magician rules is his heritage: Desjardins is the great-nephew of Jean-François Champollion, the man who deciphered hieroglyphics using the Rosetta Stone. He was the first non-magician to unleash ancient Egyptian magic, and because of that, his descendants were trained as magicians. The result is that "Desjardins is very proud of his family…but a little sensitive too, because he's such a newcomer" (18.197).
As soon as the kids figure out that Set needed a mortal host, they wonder whether it might be Desjardins. After all, the magician is arrogant and quick to anger, just like Set. During their conversation with Iskandar, for instance, Sadie remarks that Bast had said magicians were paranoid. Desjardins reacts like this: "The magician clenched his fists, and the air tingled with the weird smell of ozone, like during a thunderstorm" (14.81). Temper, temper.
In one of Carter's visions, he sees Set ambushing Amos. Set greets Amos with bonsoir, the French phrase for good evening. Now, that's bad news. Carter puts the puzzle pieces together: "If Desjardins hated our family, and if gods tended to find hosts who shared their goals, then it made total sense that Set would try to merge with him. Both wanted power, both were resentful and angry, both wanted to smash Sadie and me to a pulp" (18.202). Seems pretty logical to draw a connection there.
However, when Desjardins is flying with Zia and a handful of other magicians to the U.S. to take on Set and the godlings, their plane goes down in a storm manufactured by Set. Desjardins orders the magicians: "Protect the innocents!" (26.105). That's not exactly what you'd expect to hear from an incarnation of a destructive god, is it? Maybe there's more to Desjardins than meets the eye.
Oh, and by the way, his first name is Michel (pronounced like "Michelle," because he's French). That's not what we expected either.