In Vatican Miracle Examiner, the main characters are often addressed as Fathers Roberto and Hiraga. Thus, Roberto is addressed by his personal name, whereas Hiraga is addressed by his surname. (Hiraga's full name, using English-style ordering, is Josef Kou Hiraga. Josef seems like a name taken upon entering the priesthood.)
Why is there such a discrepancy? What I've seen in the past (e.g. in The Brothers Karamazov) suggests that at the very least, it's not unusual for priests to be addressed as Father + $personal_name, and Hiraga's form of address does not fit that.