This guy might be a minorish character, but he plays a hugely significant role in Sentimental Education, so keep your eye on him.
Here's what we know about him: he's a math teacher, a sort-of friend to Frederick, and a friend a friend to Deslauriers. He starts out as a hardcore Republican (read: he wants the king out) and a Socialist (read: he believes in economic equality among the people and overthrowing the rich, corrupt, materialistic establishment).
Check out this early description:
[H]e had his teeth daily set on edge by the announcements in L'Art Industriel. Arnoux was for him the representative of a world which he considered fatal to democracy. An austere Republican, he suspected that there was something corrupt in every form of elegance, and the more so as he wanted nothing and was inflexible in his integrity. (1.5.33)
Here we have a guy who just won't stop his fist pumping about politics—and Frederick isn't a fan. Not surprisingly, Sénécal gets majorly involved in the June 1848 revolution of the working class and eventually goes to prison because of it.
And that's when things get interesting. Yep. When he gets out of prison, he suddenly starts supporting Louis Napoleon III and becomes a so-called Bonapartist—someone who supports restoring empire. Um, didn't we just get rid of that? He even starts spouting crazy propaganda in favor of tyranny.
The last time we see him, Sénécal is working under Bonapart as a mounted policeman, murdering his friend, Dussardier, who is protesting Louis Napoleon's power. Looks like the tables have turned. But what exactly happened?