Isn't it odd how the yellow SOUL, which is based on justice, is used in the battle against Spamton NEO, a crooked and backstabbing salesman who stole the power of the NEO Armor? Regardless of if you fight him or spare him, he falls regardless and is stopped, bringing him to justice in a sense. What if each bonus boss's personality/fight is based on the opposite of what their soul mode represents?

You could apply this to Jevil too, since the red souls isn't attached to anything in particular, so Jevil wouldn't really have to be an opposite. Thouhh, if you WERE to go that route, given how the red soul is used to order you and your party, which is emphasized in the snowgrave route too, you could argue that Jevil's personality being chaos, or A LACK OF ORDER, would be the counter to that. And regardless of how you defeat him, he's brought into some form of order, whether by being your weapon, or going to rest for a century.

So does this mean that say, a Light Blue SOUL boss would be impatient and hotheaded, or maybe a Green SOUL boss, unkind and rude, and so on? Who knows really, but it is an interesting thing to think about.