Perhaps Adrian overhears a conversation or stumbles upon a secret document that uncovers the truth about Ladybug. Another possibility is that a villain forces the reveal in a dramatic turn of events.
He could be shocked at first. I mean, he has been so close to Marinette all this time without realizing she was Ladybug. His reaction might include disbelief and then a quick acceptance as he starts to piece things together in his mind.
Well, in some fanfics, Naruto stumbles upon old records or meets a mysterious elder who reveals the truth.
It usually happens through some unexpected revelations or quests. Maybe he stumbles upon old records or meets a mysterious figure who knows the truth.
Sometimes, it could be through a magical reveal or a series of clues that he pieces together.
Well, usually it happens through some kind of special event or mission that reveals hidden secrets.
I really like 'Felix's Redemption'. In this fanfic, Felix starts off as a rather antagonistic character, but as the story progresses, his relationship with Adrian evolves. It's interesting to see how they come to understand each other better, especially in the context of the Miraculous Ladybug universe with all its magic and mystery.
Perhaps in the fanfiction, Adrian's death could be a result of a self - sacrifice. He might give up his life to save Ladybug or Paris from a great danger. This would lead to a whole new set of challenges for Ladybug as she has to deal with the loss of her partner and also continue to protect the city without him. His death could also be a catalyst for a new hero to emerge, or for Ladybug to gain new powers out of her extreme emotions.
One popular theme is family secrets. Given their family background in the Miraculous Ladybug world, fanfictions often explore the secrets that Felix and Adrian might be hiding from each other or from the rest of the world. For example, there could be secrets related to their parents' past or the true nature of their family business.
Marinette (as Ladybug) would be devastated. She might isolate herself at first, blaming herself for not being able to prevent it. Alya would be very concerned for Marinette and try to comfort her, but also be in shock at the loss of Chat Noir. Nino would probably be heartbroken as well, since he was friends with Adrian. The other classmates might be sad and confused, and there could be a general sense of gloom over the school.
One interesting plot could be Adrian's initial shock and then his realization of all the times Marinette's behavior as Ladybug made sense in her civilian form. For example, her courage as Ladybug might be seen as her standing up to Chloe in her Marinette guise.