He's a lazy, pun loving skeleton.
That's exactly what Oblivion's canon form is. However, while he is lazy and playful, he also scans for weaknesses and offenses he considers to "Judge" for. He does make puns throughout that forshadow his transformation, but people just throw it off just because he is regular sans.
Then the fun happens. He turns into this unbeatable judgemental godlike figure (Oblivion form) that criticizes people on what he found based on the character. He exploits those weaknesses and vulnerabilities as a way to mentally torment his opponent. He either fires his gaster blaster with perfect accuracy, obliterating them, or he fires a near miss and continues to manipulate the opponent out of fear.
Caption this Meme 