Souls reflect moral character as beauty or ugliness
Basically, whenever a character's Soul is seen, it is proportionally as beautiful or ugly as their character and morality. Most of the time, whenever a character's soul is rendered visible (be it through Aura Vision or via Astral Projection) their soul look almost identical to their physical self, albeit semi-transparent. However, much like The Mirror Shows Your True Self and Morphic Resonance for Shape Shifters, the truly righteous and the hellishly vile will have their souls take on angelic or demonic appearances, even if they don't have mystic powers. This being based on a character's morality rather than appearance, it's often used to juxtapose how inner and outer beauty can be wildly different. That guy with the Face of a Thug and a freaky fashion sense is actually nice, to the point his soul is just a pair of wings and Holy Halo shy of an angel. That lovely and lascivious Daddy's Little Villain? Anyone looking at her soul would have to fight to keep their lunch down. Now, if the character in question has some kind of (un)holy or mystic powers, then their Soul Self will take this Up to Eleven and be just shy of true divinity. In fact, ghosts often employ this in their "base" form (since some can often take different forms), with evil ones looking like amalgamated hellish clouds, while others resemble a benevolent Guardian Entity. A victim of a Baleful Polymorph or character with Voluntary Shapeshifting may have their disguise seen through this way, for good or ill. There's also a self-image component to this trope, where a person who prefers themselves as young, or imagines themselves as prettier/uglier, may take on that form as a soul. Contrast Jacob Marley Apparel.
Examples:Anime and Manga