It returns an int so it can return -1 if there's no character available. If you know there is (e.g. because you just called Serial.available()), you can simply assign it to a char, e.g.
char c;
if (Serial.available())
{
c = Serial.read();
}
It's perhaps slightly less efficient, but not significantly (e.g. compared with the time it take to transmit the data over the serial port).