Van is the son of the (ex)pope and he was also one of Yuuri's (ex)love interest. Yuuri's the heroine that causes trouble for Lady Iris, and Yuuri left after the fall of his family. Van hasn't gotten much screen time but among the capture target's he seems like an airhead upstart; the church carries the same weight as the royal family and for a panel they mention inquiry about his family's legitimacy but that's trivial? 
But the reasons aside from being Yuuri's possy is that his father cooperated with other parties during the trial arc to take down the Amelia family. He attested that Iris destroyed a church without their permission, he later got imprisoned after. Particular reason for that plot, the pope was making a deal with another country through a third party because he wanted to overthrow the sovereignty and the promise of prestige in another country.