Bolton Miller, is a detective who works alongside Will Bowman, they seem to work well together.
Before The Arrival Beau was a detective with the San Francisco Police Dept. By chance he was in L.A. on the day of The Arrival and was stranded there.
When Will started work at Homeland Security he appeared to be disinterested in his job, after a while Will's professionalism rubbed off on him a little and he started to both value Will as a partner and to work hard in his role as a detective.
Will clearly confided in Beau, and Beau supported Will in his attempt to get his family out of L.A. by offering the use of his property in Big Bear. On the night when Beau, Will and the Bowman family were to escape the L.A. bloc, Katie refused to go and Beau had to leave on his own.
Beau's current location is unknown, but it seems likely that he left through the Dept of Water and Power tunnel and is now at his property in Big Bear or he was transferred.