Father Clifford

 is the head of Saint Monica's Church in Rivet City. His acolyte is Diego. He is normally seen in and around Saint Monica's Church but is known to roam around the inside of Rivet City talking to other residents.

If you talk to Father Clifford he gives you a brief history of his church and role.

Every Sunday Father Clifford holds a mass, for which most of Rivet City's named NPCs show up.

Unmarked quest: Church Donations
If you want to gain karma you can talk to Father Clifford and make a donation to the church.

Bugs
The Sunday sermon is often glitched: either Father Clifford is not present at all, but NPCs shush each other because he is speaking, or he will be present, but not speaking. It is advisable to go to church before starting the Marriage minor quest, as the Sunday sermon seems to get stuck because of a bug in that quest.

Resetting Father Clifford may allow the sermon to continue, or at least allow the NPCs to stand up and continue their daily business.

He has also been noticed speaking his sermon in the upper deck hallways on days other than sunday.

Appearances
 only appears in Fallout 3.