Felix isn't going to betray anyone or turn evil again.
This isn't a theory or a hypothesis; this is something I can guarantee and assure you without the slightest doubt.
Why do I say this? Because Thomas confirmed that Felix already apologized for everything he did, which happened off-camera at the pool party at the end of season 5. And we can corroborate this with the fact that he felt guilty when they mentioned the yo-yo theft in Yaksi Gozen, showing that they had already discussed the topic before.
Now, why would this be proof that he'll never betray anyone again? Because Thomas said there wouldn't even be a flashback because, and I quote, "it's not important to show that."
"But why would you believe him? He himself said that Chloe wouldn't come back, and there's the last episode!" And you're absolutely right, he lies through his teeth, BUT, this time he's right. He gave the same answer regarding Lila getting the Butterfly Miraculous: "It's not important, we don't need to see it on screen." Lila is already the villain; she's already akumatizing people. From the beginning, they've been operating with the mystery of what she does, what her objectives are, and what she's trying to achieve. Seeing how she gets the Miraculous is irrelevant to what they want to show.
The same goes for Félix. He's already one of the good guys. He's already Adrien's soft-uwu-boy cousin who loves his family. He's already a lovestruck little bird with Kagami. He's already a useless idiot who hasn't done absolutely anything in 24 episodes despite being one of the good guys now.
Neither an apology nor redemption is relevant; he's already on the other side. And that's proof that it's not going to change in the future, because the only times they put in the effort, the only times we waste time again and again trying to change a character, to show some growth, some progress, is to make a 180-degree turn and surprise the viewer.
They did it with Chloe, they did it every time Gabriel seemed to remember he loved Adrien, and they even do it with storylines themselves, like freeing Adrien from his father's cage only to put him in a new one with the big lie. The more effort they put into it, the more screen time they give it, the more time they WASTE with it, the more sure you can get that it's going to end up the same as it started.
So yes, in Miraculous, change only happens off-screen.
