Folse is pretty much a pitifully submissive character. As the 3rd son of his family, he was pretty much a spare. Plus he'd no strong desire for power or control, simply preferring to follow the orders of others. He also had no chance of inheriting Rozalia's family marquisdom by marriage, since family rules dictate only closer male family relations can become the head (ex. Alexis, because he's Rozalia's blood-related cousin).
It should also be noted the king was the one who ordered that Folse be married to Rozalia, despite his family's misgivings.