I got this idea based on Hartmut from Ascendance of Bookworm, When I think about how obsessed and good at planning Marinette could be, this pops into my mind. I had the idea that Marinette was the mastermind between Lila for some time, so I merged those together.

Alethea can still remember every detail of her first meeting with Hades Heinstein, the then Heir of the Heinstein family like it was yesterday. It had been at an engineering competition that she had been invited to by a friend of hers. Honestly, it still amazes her to this day that no one knows she had friends outside of school. The competition had hosted by a branch company of the Heinstein family. She couldn't remember what the projects were as her entire mind was filled with Hades' opening speech. She can remember every word of that glorious speech about pushing themselves to be best and not letting anyone or anything stop her or hold her back, not even friends or family. She had never felt such intense love before, not for her parents, not for her friends, not even for Adrien. It was such a different love too. Not familial love, not platonic, not even romantic. It had been the love and devotion towards someone who was a god. She had wanted to serve him from that moment. It hadn't just been his confidence and ruthlessness, but also his noble bearing. They have the same black hair, blue eyes, and pale skin, but why does it look so regal on him?

Back then she was still weak Marinette. She had no idea what to do to get his attention. However, he knows of her interest in serving him. Barely a week after the competition, she had found a letter in her sketchbook with the Heinstein crest. It read "Your ability in fashion is more than enough. However, what about your loyalty? Prove yourself. Destroy everyone you hold dear." She still has the letter to this day. It is one of her most prized possessions to this day. She hadn't hesitated for a second. Her mind churns with plans.

She had decided that she needed something that takes out several people all at once. The idea had come to her in a flash as she was toying with a necklace her grandmother had given her. She had overheard Gina telling her parents about her best friend's daughter who had been ostracized and bullied when a classmate spread lies about her. A few true friends had stood by her, but it hadn't been enough to stop her from committing suicide. Her grandmother's friend had pressed charges, but since the classmate's mother was a diplomat. If she wasn't wrong, the name was Lila Rossi. (That was the original reason why she hated lies. Now it is because the truth is stronger than lies.)

She started to do research on her in internet cafes (to avoid leaving a trail. She had read a lot of mystery and crime books. Not the best source, but one thing was clear. Cash is untraceable.) It seems that Lila Rossi's modus operandi is making false lies about knowing celebrities and promising favors. She accuses anyone who calls her out of her bullying her, and getting their classmates and friends to turn on them. How that works out varies. Some classes see through her lies immediately, calling her out and keeping their distance. Others fall for her plots and turn on their former friends. She wonders which way her class will go.

Marinette didn't have the power to arrange for Lila to come to Paris. However, Chloe Bourgeois did. Getting a meeting with her was the hard part. She ended up using her Ladybug identity to do so. She tempted Chloe with the bee miraculous, saying she will give it to her if she meets Marinette in her penthouse. Marinette arrives at the hotel in disguise.

Marinette easily convinced Chloe to help her. She claimed she was tired of the class asking her for favors and want to see if they were using her. She tempted Chloe with the fact that after a few months, she can stand in front of the class and give a presentation on Lila's history, humiliating them. She sweetened the deal with the fact she will give up on Adrien. (It wasn't like it was that bad of a concession. Compared to Hades, he was just bland.)

Chloe did her part and that was when the fun begins. Marinette purposely call out Lila and turned the latter's wrath on her. Things happened as she predicted. She was disgusted by how well it worked. Her class, the people who know her for years, turned on her accusing her of bullying poor Lila. They thought she was capable of being a bully! Granted she now know she was capable of far worse, but still. At least the other classes didn't believe Lila. She had made sure to drop hints about Lila's stories and how ridiculous they are in the earshot of the biggest gossip in school.

What she didn't expect was for her own parents to fall for Lila's lies. They punish her and scold her. Honesty with how incompetent the school staff is, why would they trust anything they said? Hades had comforted her saying that parents don't always unconditionally love their children. He tells her about employees whose parents had stopped loving them because of their sexual orientation. He mentions that some of them were kicked as soon as they turned 18, which set off warning bells in her head. She immediately started to move money from commissions into another bank account. She had needed to become Ladybug to open the account, but the bank manager was very helpful especially when she made a public appearance. Her parents' actions had hurt at the time, but it was better she saw their true selves now than later.

It had been quite entertaining when Chloe at the end-of-year assembly (which she had demanded) revealed everything about Lila's past and the truth about her lies. Their class had turned against Lila, questioning her. The other classes altered incredulous at Ms. Bustier's class's stupidity and disgusted at Lila's past. Chloe managed to surprise her and catch a video of Lila taking an Akuma butterfly. The presentation ends with police coming in to arrest Lila. Marinette and Chloe had sneaked off to transform. They managed to catch the butterfly before Lila can reach it. Lila had been taken away by the police.

Marinette hadn't returned to the bakery that day. She had received a message from Hades with an invitation to join his service. It had been hidden in her bag. There was a plane ticket to go to Germany, as well as instructions on how to get an apartment in a safe part of Berlin. Admittance to a well-known school for fashion was included. There was several hundred euro for her to treat herself in Berlin.

In Chloe's penthouse, Chloe gives Marinette the emancipation papers she had asked for. Marinette give her the Ladybug Miraculous to use now and warned her to find another holder immediately as Chat Noir had been nonexistent ever since Ladybug had declared that she prefers cold, mysterious men.

"So, you are abandoning everything, even your miraculous, to serve him?"

"Yes."

Chloe said nothing in response at first. "Then, I wish you luck. Keep in touch." Marinette left Paris that day and she never once look back or regret it.

Germany becomes her home. Marinette works harder than ever. When she graduated and enter Hades' service, she took on a new name, Alethea, Truth, for it was the most powerful thing in the world. She built her own fashion company with Heinstein's wealth and influence helping her. In time, she wed Elisabeth Schneider, a woman from a family who have a tradition of serving the Heinstein family. The match was perfect because Elisabeth was a good businesswoman and just as devoted to Hades as her. Their marriage was happy.

Alethea had little to do with Marinette. When she visits France, she never meets any people from that time. She was always focused on work. Many of the relationships she had faded with distance and time. Her few friends, (Kagami, Luka, and Chloe), were either in other countries or too high in the social ranks for her to meet anyone. If she does happen to run into anyone from her old class, she acts as if she doesn't know them. Alethea doesn't know them. She usually drives them off with her tongue. From Chloe, she learned that their former classes had gone downhill because they missed many opportunities due to Lila's lies. The other classes have also refused to help or have hindered them as it turns out Marinette wasn't the only one, they were bullied because of Lila.

During one of her trips to France, she helped Scarlet Lady (Ladybug Chloe) defeat Hawkmoth, who was Gabriel. Gabriel had been discovered when one of his designers attempted to steal a rival designer's work. The rival designer located an investigation that led to Gabriel being exposed. Adrien comes under heavy fire. He tried to protect himself by saying he was Chat Noir. However, that backfires as Scarlet Lady's first action was to remove the ring from Char Noir because of his terrible behaviors. No one could be sure if he telling the truth and even if he was, he was more of a hindrance than a hero.

Gina had a falling out with Tom and Sabine. Marinette had sent her a copy of the video Sabrina of Lila's downfall. (Chloe had wanted to send it to her mother. From what she gathers, Audrey praises Chloe for it, calling her exceptional.) Gina had shown it to her friend, who had bounced back from her depression (caused by the death of her daughter.) She asked Gina to give Marinette a gift from her. Gina had gone to the bakery to give it to her and told her son and daughter-in-law that she was so proud of Marinette for putting that horrible girl Lila away. Tom and Sabine immediately protested saying that Marinette was the one bullying "poor Lila." Gina had seen red as she figured out instantly what had happened. She had screamed bloody murder at them about Lila's past, before demanding to see Marinette, only to learn they don't know where she is. Gina had left, but before ripping into them about their incompetence both as parents and adults. She finds Marinette easily enough as her face was all over Germany since her fashion is the leading brand now. Gina accepts Marinette's new name and life. They keep in touch, until Gina's death.

Tom and Sabine never really recover from the relationship with Gina. Not just for how they treated Marinette, but for believing in Lila. The relationship did start to mend when they had another daughter Bridgette. However, Gina dies a few years later. Alethea attends her funeral. That was the first and only time she saw the Dupain-Chengs again. Not that they know it was her due to the heavy veil she was wearing, and Elisabeth block their view of her. She didn't even realize they had another child until she was contacted by the French CPS about custody of Bridgette.

"We can't take her," Alethea said firmly. Elisabeth nods in support, her Ice Queen mask glaring at the poor man.

"Why? You have the resources to take care of a child! You own an international company."

"Emphasis on international. I can spend up to half the year in other countries."

"What about your wife?"

Elisabeth said haughtily "I don't want to parent a child that isn't mine. Besides we are going to have a child of our own soon." The CPS worker gapes the two women. "A new experimental procedure has been announced by the Heinstein Corp, where they will merge the DNA from our eggs and insert it into Alethea's womb. If that doesn't work, my brother will donate sperm. Either way, we will have a child of our own."

"Check with Chef Wang in China."

"You are both assholes!"

"I will be telling your boss you call us that."

"I am going to be bed rest and observed by doctors for months, so there is no way any court will agree to this."

Damage control had been easy since the procedure was already started. Barely a week after the meeting, as scheduled, the fertilized egg was implanted in Alethea's womb, and she was put on bed rest. 9 months later, Alexandra Schneider was born. As she holds her daughter in her arms, she feels pure happiness. Her life was perfect. If time was turned back, she would do it everything the same way again.

Gina was deeply affected by what happened to her friend. She talks about it a lot to Tom and Sabine, so she was upset they didn't remember. Gina is very proud of her granddaughter and approved of her actions toward Lila. Marinette could have committed murder toward Lila and Gina would still have approved. Heavens she would have provided an alibi.
Ladybug Chloe is from the Scarlet Lady webcomic.
Schneider means tailor.