I don't know where to start or how to succintly describe him in a way that properly captures the vibes especially if you don't have a coherent view of the state of French society.

Trotskyist background, leader of the biggest party on the left (which is also the party that's furthest to the left if you only include electoralist parties). Founded said party because the socialist party was too reformist and cozying up with the center/center-right. Likes quoting Lenin and insulting capitalists. Even though he clearly believes electoralism is useful it's clear he has a class-conscious/anticapitalist view of the world. Is pretty well-read and good at giving speeches. His party does not have a democratic structure. People on the center/right try to villify him and say he's uniquely bad, sometimes using quotes that are actually pretty iffy. Anti-NATO, critical of the EU (which he gets plenty of criticism over by people who aren't as left-wing), anti-nuclear. Doesn't want to give weapons to Ukraine. Usually fairly well-liked among young people on the left, is usually considered to have managed to create an "actual" leftist movement à la Bernie Sanders. People on his left, those who think electoralism isn't enough etc. might say that the way the media class and the capitalists are scared of him (and form a united front in attacking and villifying him) is evidence he might actually be a threat to their interests. Part of his electoralist strategy was to try to reach out to the classe populaire in banlieues, get them to vote and all that. Any issue I didn't mention you can probably imagine what his position is.