At the start of the grander timeline, Locke is just like many regular people. She’s a normal girl who sucks at a lot and does her best when expressing her creativity. She’s not popular, but not a nobody by any means. It would be fair to call her boring.
Overall, her character is meant to represent any random innocent person. Later, you will see how she develops.
It as at this point that she finally understands that something weird is going on. She finally retains memories from a previous life, one where Modotsu had betrayed/hurt her at the very end, but in her following life Modotsu was someone close to her.
This is an extended phase that directly succeeds the previous one. This likely won’t be shown in full, and will be more of a showcase of how formulaic everything got for poor Locke. After being tortured by Modotsu until her death in her last life, Locke is almost dead inside, crying in her sleep, not fulfilling any of her obligations, and wandering aimlessly. This continues until she sees Modotsu again, and in the middle of public and out of PTSD, she screams and runs. Modotsu takes notice of this, realizing Locke must have her memories, and she begins to torture her across realities, no matter where or who she was, Locke would be caught at the peak of her life and brought right back down to 0 every time. (This was especially terrible for Locke, who always eventually found hope again and got back to her normal self.)
After what seems like forever spent undergoing abuse at the whims of Modotsu, there came a time where Locke managed to reconvene with Fanumi, who then begged and pleaded with Zenori for her to come to Locke’s defense when Modotsu came again. Zenori, wanting to appease her love decided to help out. This leads to a situation where Locke can learn about the truth of reality with Zenori and Fanumi.
After this, there comes the confrontation when Modotsu comes to end Locke’s life. Zenori, as promised, takes up the fight against Modotsu, utilizing her own power to stop time. This conflict ends up with a Modotsu dying due to carelessly assuming nobody could match her.
With Modotsu gone, Locke was able to learn much more about everything. As a result she becomes more resilient. Scared of Modotsu, but with her new found friendship with Fanumi and Zenori, knew she wasn’t going to be in any danger. This results in Modotsu & Locke having a much more consensually toxic relationship, but still not quite great either.
It is at this point that Xia will notice these changes and worry that a repeat of what happened at the start of her rule would occur. Where the Primi would have no reason not to try to understand even more, when their curiosity gets the best of them. So she takes action.
By this point in the series, everyone will have learned of Xia’s existence as well as how to access the palace. (Destroying a reality yourself with Modotsu’s powers or accelerating until it dies with Fanumi’s powers.)
Eventually there comes a pivotal moment for our protagonists after Xia decides randomly to make them suffer, showing them that when she gets bored, she may just torture all of them. As a response they decide she must go. Well all of them but Modotsu. She is willing to help them get to the palace, but not to fight Xia until they win. (A reasonable decision.)
However eventually they do get close to beating her, but its all of their last chances to do so before Xia is able to destroy the remaining universes. Modotsu changed her mind shortly after hearing this, but due to how time works at the palace, she ended up coming too late, Locke had been killed.