Frostburn by Niall Teasdale

As New Millennium City tries to recover from a violent summer, a new rash of terrors hits the streets.

An arsonist is at work in the more affluent areas of the city, destroying property and ensuring that people die in his fires. Fire Bug seeks the help of Cygnus, Svetilo, and even Twilight to fight the flames and save lives, but it’s hard to keep up with someone who seems to be able to go anywhere he wishes.

Meanwhile, in Deale Harbour, an icy death awaits those on the streets at night as a killer stalks the night freezing his victims to death. Twilight is on the case but there is an aspect to it which worries her: her boyfriend, Jacob, has been exhibiting random bursts of Ultrahuman power, power which lowers the temperature around him. Could Jacob by the man responsible for the frozen corpses mounting up in the morgue?