Billy here is a Cupid, meaning that he can make people fall in love with just the use of an arrow.

Most of the time he uses it for the right reasons - if a friend of his looks like they're struggling with a girl and he knows there's a mutual interest, he has been known to help speed things up. Similarly, if he thinks someone is in need of some love, he'll happily help them out.

As part of the Night Watch, his arrows are often used to confuse the enemy by tricking them into falling in love with either one of their comrades, or something else (it depends on his mood). Some arrows however have been known to miss their intended target(s) and hit someone unsuspecting, though with some unfortunate results.