Well, fire is a major player in my novel, so I'm going to approach this a different way, and try to justify why each main character is like fire.
Agony- spits out sparks that burn, makes it clear that you shouldn't try to get too close, produces smoke
Thesis - can be comforting, but also annoying to be around. reminds you vaguely of living rooms
Duenna- will take care of you, but prolonged exposure to her will cause you pain.
Motif- unpredictable. always burning himself out. can produce energy, but surrounded with the waste product from doing so.
Sequel - a fire burning low, it's evident it burned bright at once point, but it doesn't now. Maybe it will again if given the right fuel.