Jaune Arc was always one for the supernatural, always claiming that he saw things that weren't there. He was often claimed to be seeking attention, like the Boy Who Cried Beowolf. In reality, though, he was psychic and saw into the spiritual world without any aid.
When he first got his Aura, they appeared. Thirteen gruesome spirits with tragic and horrifying backstories appeared before him, though no one else could see them.
He saw that, while they had their birth names, they went by totally different titles.
There was The Juggernaut, The Torso, The Hammer, The Jackal, The Pilgrimess, The Angry Princess, The Dire Mother, The Great Child, The First Born Son, The Torn Prince, The Bound Woman, The Withered Lover, and The Broken Heart (1).
Each were gruesome in appearance, while most were very dangerous. They, however, never attacked anyone, unless they are provoked or someone threatens Jaune. The Grimm, however, were a different story.
Every since the spirits came, the Grimm had given the little city of Arcadia a wide birth, as many of their kind were brutally destroyed and mutilated by the ghosts if they tried to attack the city.
Everyone assumed it was Jaune's semblance, something so powerful that it made him forget he every used it in the first place. This, however, was false. Jaune's semblance was very passive.
He could see the aura of others, like the health of an enemy or ally in a video game.
~~
Years had passed and the Thirteen, as he called them, became even stronger.
He claimed that they were his semblance, to sorta explain the strange happenings around him caused by the ghosts, and was accepted into Beacon when he was seventeen.
Look out, Beacon. The Dark Zodiac is coming.
~~~
(1) The Broken Heart wasn't actually in the canon of the movie, but, for the sake of the story, he came to Remnant shortly after the movie when he died from blood loss after losing a foot - again, something that isn't canon - and appears with the others.