Very remorseful person. They probably have a traumatic past or feel guilty about something they've done. They're also clearly very sarcastic and they're trying to hide their sadness behind humour, which resulted in an exaggerated plumpened appearance.
The Pyro is a pyromaniac class of indeterminate origin with a muffled voice and a burning passion for all things fire related. As shown in Meet the Pyro, the Pyro may be delusional, visualizing themself in a utopian fantasy world known as Pyroland.