Flame, like PJ was an accidental child. He is named "The Unknown Child", because Sue and Ted didn't even know he existed. Flame was kept as a secret and was hidden. Lux and Lucas didn't discard him in any way, instead they kept him, but after a while, Flame became more aware of everything and wanted to see Sue for the first time(he was 2, meaning Sue was 6), Lucas.. didn't really agree on this considering what happened to her(F**K YOU SHATTERED). Flame was upset by this, and disappeared. Years later he returned to his own family and not only got to see his parents, but Sue AND Ted. Flame thought he was being replaced(which he wasn't. Poor boy-), so he left forever. No-one really knows what's going on with him unless your PJ.
PJ takes care of him in his own Void(The Anti-Sphere), where he treats him as his own.