Warning: The following contains SPOILERS for Young Justice, season 4, episode 11, "Teg Ydaer!"
Young Justice: Phantoms finally revealed the dark secret that drove Mary Bromfield, a.k.a. Mary Marvel, to seek training as a sorceress. This revelation came about during a psychic battle in which Mary's worthiness was tested by the Lord of Order, Nabu. The test found Mary fighting her superhero self, Sergeant Marvel, who resented being trapped within Mary, as she learned how to call upon the power of the gods who had blessed her using spells instead of the magic word "Shazam!"
Young Justice season 4, episode 11, "Teg Ydaer!," found the world imperiled by the continuing battle between the Chaos Lords known as Child and Klarion the Witch Boy. While Vandal Savage prepared to evacuate himself and his allies from the Earth, Zatanna, the Phantom Stranger, and Jason Blood sought to gather Earth's most powerful magicians to undo the damage wrought by the Chaos Lords' battle. Unfortunately, Zatanna ran into a roadblock when she asked the Lord of Order, Nabu, a.k.a. Doctor Fate, for help, as he was content to let the Chaos Lords fight it out and cared little for the damage they might inflict on humanity. When Zatanna insisted she and her apprentices, which included Mary, could stop the Chaos Lords with Fate's help, Nabu forced each apprentice into a battle with their own inner demons to prove their readiness.
For Mary, the test took the form of a fight with her superhero alter ego, Sergeant Marvel. Previous episodes of Young Justice had hinted at Mary losing control in the past, suggesting an adaptation of Countdown to Final Crisis was in the works. The comic book storyline, widely regarded as the worst story ever written to feature Mary as a character, detailed the young heroine being corrupted by the power of Black Adam and then becoming a servant of Darkseid. Thankfully, it was revealed that the Mary Marvel of Earth-16 was only corrupted by the power a young child might feel being able to transform into a superpowered adult with few limits and not some alien warlord or mystic power source. Her secret was dark but understandable: an addiction to life as her superhero alter ego, Sergeant Marvel.
The test found Mary facing her childhood self, who tried to provoke Mary into saying the name of the wizard Shazam to save her instead of using her spells. The child Mary finally said the name herself, transforming into Sergeant Marvel and administering a brutal beatdown to the real Mary. Sergeant Marvel mocked Mary for being "afraid to be your true self, and without enough imagination to become something new," questioning why Mary would ever want to give up being her.
Mary was ultimately saved by a manifestation of Billy Batson, who reminded her that she had the strength to give up being Sergeant Marvel after spending a period of time only living as a superhero and not allowing herself to just be one version of Mary Bromfield. This gave Mary the resolve she needed to banish the Sergeant Marvel phantasm, declaring that she could be "just as important and powerful as you ever were, without abandoning who I am." While Young Justice: Phantoms is far from the sort of animated series that preaches positive educational lessons to young children, the message of being confident in one's abilities and true to your inner self is one that might benefit people of all ages.