There are many of theories involved in Scary Godmother lore but there is only one that has been proven. The Red Color Theory has been proven multiple time by many of characters.
The main idea of the Red Color Theory is that the boy will follow who he believes is best. He will do anything and everything to prove he is worth of the best. Many times the boy will go against what he actually believes just to gain support and love from what he believes is best.
The one described as the best as many forms but the form the boy loves the most is his Slayer form (for the sake of of less confusion the best will be referenced as "Slayer").
The boy is unknown to the lore of Scary Godmother lore, this is because he hated by many secretly and if his identity was revealed all hell will break loose and the true anime arc of The Fall of Man will begin.
The boy has tried again and again to be accepted by Slayer but Slayer still rejects him. The boy is very persistent which is the worst part of this situation.
Many people support this theory, some people even add to this theory such as Jimmy.
Jimmy ships The boy x Slayer