In a 'Miraculous Ladybug' thief OC fanfiction, the thief could be a character who is initially seen as a villain but is actually a misunderstood anti - hero. They might be stealing to draw out Hawk Moth. The OC could have their own unique set of skills and a special weapon or tool that helps them in their thievery. They could leave behind clues for Ladybug and Chat Noir, not realizing that they are actually helping the heroes in their fight against the real villains in Paris.
One idea for a 'Miraculous Ladybug' thief OC fanfiction could be that the thief OC is stealing artifacts that are actually magical items related to the Miraculouses. The OC could be doing it for a noble cause, like trying to prevent a greater evil from getting their hands on them. Maybe they have a tragic backstory that led them to this path. For example, their family was wronged by someone who wanted to use the Miraculous for bad, and now they are trying to keep the artifacts safe by stealing them from those who would misuse them.
The thief OC in the 'Miraculous Ladybug' fanfiction could be from another city or dimension. They come to Paris because they heard about the Miraculouses and think they can use their power to save their own world. However, they don't understand the balance that Ladybug and Chat Noir are trying to maintain. So, they start stealing items related to the Miraculouses. As the story progresses, they could learn the importance of working with the heroes and eventually become an ally, after some clashes and misunderstandings with Ladybug and Chat Noir.
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Chapter 604, "Shiguang, originally named Shen Guang", is the clearest early answer because it turns Seeing Jiang Mingyu's situation into a concrete plot event. It opens with Fortunately, the other person did not recognize him; they just stared...
Chapter 542, "He is My Ideal Type", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event.
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If you want the clearest chapter reference, go to Chapter 2. Early on, On the top floor of the Financial Building, a gathering of dark clouds...; later, the chapter leans on How dare Jiang Li hang up his phone?.
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