People take Clair a little too personally (ironically, her flaw is taking things too personally). She hypes herself up as the world's greatest dragon-type master and someone who can hold her own against the Pokemon League. Then she loses to some random kid on a beat. Obviously not used to losing to anyone but Lance, she tries to salvage her pride and self-image by forcing you to complete a ritual trial in order to prove your win wasn't just a fluke and that you're truly worthy of advancing.
Now I don't think the trial in GS is all that good. Clair may be more honorable here because she doesn't assign you a task she herself couldn't complete and keeps her promise without complaint, but she essentially gets away with her behavior, like her only fault was her questioning of your worthiness rather than denying you something you're owed. In Crystal, she does give you a task she expects you to fail at because she's never passed it herself. But it backfires, you do pass it, meaning you've beaten her twice and Clair is confronted with the fact you may just be better than her in more ways than one, leaving her incredulous. Then her grandpa chews her out about it and being a brat, even threatens to tell Lance-oniichan, the true bestest, if she doesn't do the right thing, which causes her to immediately comply. It's actually pretty funny, she's being dressed down for behaving less maturely than an actual child. She got rekt hard.
But the best part for me is that she goes out of her way to apologize to you. She knew what she did was wrong but didn't hide after being shamed and embarrassed, instead she went to face you and make amends by giving you the TM with the ironic name and helpful info. But because she's Clair, she can't be nice without being all tsundere, which is cute. Also can't help making your Pokemon League challenge about herself, you have to beat her cousin so she'll feel better about losing to you. You're fighting for her sake whether you like it or not!
