Spencer Roane
I looked up Spencer Roane and discovered that he studied law at William and Mary, then went on to become a justice in the Supreme Court of Virginia. In a case regarding the incorporation of taxation in relation to religion (specifically Christianity), he opposed the involvement of religion in legislation, despite being a Christian himself. Continuing his advocacy for secular policy, his efforts alongside Jefferson and Madison led to the creation of Establishment of Religious Freedom in 1785. Also, he was many years younger than the other men.
His assistance on secularism in legislation translates to UVA's foundation as a nondenominational institution. This was an important ideal for Jefferson, so it makes sense that he would like Roane on the team.