So Clair's mother is actually the missing princess of a kingdom that's since been destroyed. Unknown to Clair who gets baptized in her actual homeland, she obtains the rainbow aura magic (basically a physical god). The fact that she got pink magic from a place that was not her homeland was actually a clue to her original super powerful magic. Note that she was already under the protection of the neighboring prince at this point so as the cliché goes, is smitten with her. She spends time in the new prince's kingdom and shows off that awesome JAPANESE HUMILITY, GRACE AND ALL AROUND GOODNESS.
Anyways, we jump back at certain times to prince asshat and thotlte. Being a spoiled asshole Charlotte is basically unfit to be queen and servant behind her back constantly compares her to her sister. Charlotte originally just wanted to sit in the seat of the queen but have Clair do all the actual queen duties but Clair running away forced her to have to actually do the job.
Anyways, a disaster hits the neighboring kingdom, a magic tornado. Clair, using her ALPHA PLUS SUPER MAGIC stops the tornado but uses too much magic and goes into a coma. Inside said coma she is her original Japanese self and talking about the game with her friend and discusses some route and hidden route details with said friend. <- This will be important later.
Anyways, thanks to her achievement the king basically takes the time to hand her a title so she can be official engaged to prince-chan and she's all happy and stuff. Since they know she's the Duke's daughter they return to her home kingdom both to announce their engagement and to pay respects to the king and her family. After some wish wash we meet prince asshat and thotlte and prince asshat is clearly having regrets. Anyways asshat kingdom and her father now want her back because they realized that she stopped the magic tornado and therefore, has super powerful magic so they refuse to bless and allow her engagement to go through. It is revealed that Charlotte was using charm magic to make everyone love her which doesn't work on Clair's prince and his entourage because they tool measures against stuff like that. Charlotte wants new handsomer, better prince and tries to use magic but is blocked by one of the Prince's aids who pretty much gets crippled by this. Clair reveals her rainbow level magic by using a teleport spell to retreat with the prince and his aids in tow. Following that king asshat issues and order for the return of Clair and both kingdoms are ramping up for war. Prince asshat felt so ashamed that he helps nice prince's remaining aids to return. Clair being upset about the whole war thing, remembers that she can return to her past self and start a different route. So she burns her magic and goes back.
She starts the hidden route and using her knowledge of events to come, she arranges everything so that war is averted. She marries nice prince, Charlotte is sent to a monetary, prince asshat is married to someone else that isn't incompetent and everyone lived happily ever after. The End.