L'AuBC

Feuilly is an artist, but he does it for work. I imagine, since he's probably not had a chance to explore creating art outside of a business setting, he views the process as a means to an end rather than any creative outlet (especially since, according to how fan painting worked at the time, he was likely just painting over already existing guides).

Prouvaire is a poet. He's a wealthy, only son, so it's got nothing to do with money - he does it out of love for the art. Pulling another headcanon into this, I imagine he actually has to go out of his way to even have and enjoy this hobby, but that's a headcanon for another time. The point is, he enjoys the arts as they are, free from any business mindset, purely from the standpoint of enjoyment. (And as an aside, I've got a headcanon that he really loves arts like drawing/painting, but he's no good at it himself, so he's in awe of those who can do those things.)

So we've got someone focused on making a living, a bit more grounded there, splitting his time between work and his goal of freeing the people, vs a dreamer, with his head in the clouds, spending time in an artistic venture out of desire. Keep those differences in mind, we'll come back to that later.

They also have shared/similar traits.

Feuilly is people-oriented. The orphan that adopted the world, a boy who cares about nations outside of his own. He clearly loves people, cares about people; despite his upbringing, having a background that could harden his heart towards others, he has learned to embrace humankind.

I personally believe Prouvaire has had a rough history socially. Once again, pulling from headcanons, but someone so awkward, dressing badly, blushing at nothing - that's not exactly a socially graceful person, and I'm sure that's affected him throughout his life. Until he joined the Amis, I don't think he probably had many positive social experiences. Still, despite those interactions I imagine it's likely he had experienced, he is described as loving the people.

Obviously they both care a lot about the state of things. Prouvaire is specifically described as spending a lot of time thinking about a variety of social issues - poverty included - not just the standalone issue of creating a new form of government. He's really thinking about all the issues at hand. Feuilly is also thinking beyond that base, core goal, once again considering others outside of France. They have a wide view of what's going on.

However, I do think the way they view the issues is a little different. Feuilly, again, I think is slightly more grounded in the reality of it all, though he's highly optimistic about things, dogged in the justice for the things he cares about. Prouvaire, though, he's a Romantic. There's a lot more focus on the emotional effect of things there. He dwells on the melancholy and tragedy of the world - pitying women and weeping over children. Not to say he doesn't act, of course, he's not solely dreaming about a better future without doing anything to make a change, but there is inherently a more gentle approach to it all.

As a side note, I do think Prouvaire probably suffers from low self-esteem for multiple reasons (though, again, I'm partially diving back into headcanon territory for that thought, but I do think it would make sense for how his character is described regardless). I believe Feuilly cares a lot about people's right to identity, to be confident in who they are. Though he clearly focuses a lot on national identity, what do people consider justice for humankind without including the right of individuals to be who they are, both in a national sense and a personal sense? I imagine he'd very much want to boost Prouvaire's confidence.

And completely in the territory of headcanon, I imagine them both to be freckled redheads, but where Feuilly has a fiery red head of hair and is covered with freckles, Prouvaire is a softer strawberry blond with a smaller smattering of freckles. Zero basis in canon for that one, just how I happened to picture them and realized how charming that similarity is once I started thinking about the friendship I picture between the two. But that is neither here nor there.

The point is, I think they would really benefit each other and get along superbly. I think Feuilly's genuine, earnest attitude would reach Prouvaire's shy and uncertain heart, and I think Prouvaire's gentle love for things would give Feuilly a new perspective to appreciate. Feuilly can encourage Prouvaire to love, not just others, but himself, and Prouvaire can encourage Feuilly to find the beauty in his own work, in the art he does, even if he's technically doing it out of necessity. Where Feuilly can fire up Prouvaire and help him grow, Prouvaire can soften Feuilly and help him slow down and stop and smell the roses, so to speak. In all these ways, they make for a balance of each other, I think, and a compliment, too.