Based off the song "When she Loved Me": From Adrien's POV, him remembering their times together fondly. As they age, Ladybug is late more often to Akuma attacks (probably busy w/ her job), before she rarely comes at all. Then months later, she comes back, fighting one more time, and he's overjoyed, but at the end of the attack, she says "Bye bye little kitty" before leaving, and from then on, Ladybug is never seen again, Chat disappearing mere weeks after due to the depression of his lost love.
But what if the reason Ladybug started disappearing in the first place because Marinette had an illness that she was fighting off? That it took all her energy and she couldn’t be the hero that Tikki chose her to be. But Tikki waits for Marinette to get better, but it’s taking so long that Marinette cries herself to sleep every night wondering about Chat Noir and how she feels like he’ll never forgive her. It agonizing for her to not be able to tell Chat the truth, so she thought it would be best to say goodbye the way she did months ago. Tikki can see how upset Marinette is, and even though she has support from her best friend, she forced Alya to keep her mouth shut to everyone about what was happening to her.
But one night, when Marinette is sleeping, Tikki flies out from the hospital room, and into the afternoon. She has no idea where she is going but her goal is to find Chat Noir. It takes her hours, and Tikki knows how stressed Marinette must be without her, but when she finally spots Plagg eating cheese, she rushes over to the window and knocks against it.
Adrien is sleeping, midday, still not over the disappearance of Ladybug, but is stirred awake by the knocking. His eyes widen and opens the window for the Kwami. Tikki is reunited with Plagg for a moment, but she pushes away from him and turns to Adrien to explains EVERYTHING to him.
He doesn’t know what he should do or if he should even know of Ladybug’s identity, but when he hears that she has been sick with a life threatening illness, he wants to know everything. Adrien is a loss for words when he is told that the love of his love with Marinette, the girl who transferred schools months ago, but then he realizes that it was all a lie because of what she has been going through. For the first time in forever, Chat Noir is back and running on rooftops, following Tikki to the hospital. At this point Marinette has just spoken to the doctor. She was informed she would undergo a surgery that will determine if she will beat whatever she’s dealing with, or not. She’s crying, holding a pillow to her chest, hugging it, and missing Tikki, until the creek of her window makes her head snap up. On the windowsill, Chat Noir is looking at Marinette, eyes welling up with tears.The only thing he can do is fall to the ground, crying his eyes out, because he’s so happy to see her again. Tikki floats to the distraught Marinette and explains what she did and why she did it. But she doesn’t have time to panic before Chat Noir is standing up, hugging her. Chat Noir whispers something into Marinette’s ear that makes her crumble to fast that she would fall apart if he hadn’t been hugging her so tightly. She holds onto him for dear life while he cries just as hard as he is. Do you know what he said to her?
“We are a team, and if the illness wants to fight you, it’ll have to fight me too.”