Father Markus Kane, voiced by Doug Rand, is Lucas' older brother. When they were kids, Markus tried to get Lucas, who was something of a loner, more involved with the other children on their military base home. With time, Markus matured and now lives and works in New York as a Roman Catholic priest at St. Mary's Cathedral. When Lucas confesses his actions at the diner, Markus, despite being emotionally wracked, worries for his brother, as well as the current state of affairs; his faith and inability to comprehend what is happening to Lucas over the course of the game prevents him from seeing past his own fears for Lucas' mental health.
Source: Wikipedia, "[en.wikipedia.org/wiki/Fahrenheit_%282005_video_game%29%23Plot Fahrenheit (2005 video game)]", available under the CC-BY-SA License.