Second time watching it, the protagonist's flashbacks being represented in crayons is rather apropos when it's revealed he's the abusive father who hurt the young prince. It's meant to signify that while his son moved on without him, he never emotionally grew.
Whether this was intentional or not, the five bosses seem to represent The Five Stages Of Grief once one takes the first ending into account. The mummy is Denial because he represents a moment in his life when the Father didn't want to face how badly the fire marred his face. The pink glob represents Anger for how the Father vandalized his paintings in a fit of bitterness. The Mirror Master is Bargaining, as he adopted an "ignorance is bliss" mindset when he broke all the mirrors in the castle. The Lock Master signifies Depression, as he was locking himself off from a world he felt would never embrace his hideous face. Lastly, the Drunk King is a twisted variation of Acceptance, as he resigned himself to the bottle in order to numb the pain of his isolated existence.
Alternatively, reading the names of the achievements related to said bosses, there's an alternate interpretation. Said achievements seem to be written from the father's point of view. The mummy is the initial trauma ("Can't be real"). The pink glob is Denial (""Painful image"). The cristal humanoid is Anger ("THIS IS NOT ME!"). The lock giant is Bargaining, which can include changing your own lifestyle in the hope to avoid what's causing you grief ("I need to hide myself"). The drunken king is Depression ("Who am I?" - implying that the father's alcoholism led to him developing amnesia). There is no true Acceptance until possibly the very end of the game.
