Aerith: Flirty Extrovert or Demure Flower ?

[Sephiroth]

Perhaps she has grown in character, changed due to a dramatic life event, or just decided to act differently. I know i'm a different person now than I was in my teens.

Yes, metaphorically we are all different people in our lives and yet actually we are always us. Fitting as you said "... than I was in my teens", as both the past and present is just you with changes of character and of course the 15 year old Aerith is different from what she is with 22. Just imagine how embarassed I am of myself looking back at how I was with 5 or 10 sometimes et cetera.

Like if Aerith was a raging lunatic in CC, does that mean we have to accept that just because the game is canon? Or should we call BS on such things?

Yes, we have to accept it (or at least should) because she is not our property. She is Square Enix' property. If we make our own story that is our property. Canonicity is good because everyone being able to determine the truth would result in a paradox made of fan 1 deciding this is objectively right and fan 2 deciding that is objectively right which is why I cannot happen in the first place. We can still decide not to like canonical things. Liking/Disliking things and the acceptance of canonicity are two different things though many people mess up by mixing those two, especially when their imagination of what the fictional world seemed to be for them shattered. There are only a few exceptions when a canonical statement really cannot count and then it is pretty much still canonical but an inconsistency that is only possible to exist and does not really cause any harm - though being either a missing explanation or a contradiction - because it is a fictional world. Retconnings are completely legit, of course - but I mean out-universe retconning and not with in-universe time-stuff that does not work in the first place. That is also just nonsense (still it would be canonical but another inconsistency and so I would always just rewrite out-universe as author and never make it part of the universe by saying "this happened by it has been erased by whatever that is not possible".

Also, I agree that at times some character traits don't fit the scene. For example, in AC it wouldn't make sense for Aerith to start flirting with Cloud, but she didn't need to. She encouraged him to keep going. However, at the same time, the small snippets of her that we do see don't give me the impression that she couldn't if she wanted to. It fits with how I came to know her in FFVII.

Whih is the very reason why I don't have a problem with Advent Children. People especially critisize Cloud for being how he is in the movie but that is one of his natural sides. Cloud is a guy that only is arrogant when he feels he is in the right position to be so and is self-critical when he absolutely thinks that this is not his territory. Even while confabulating those characteristics retained on his surface. And losing a person as important as Aerith right in front of his eyes, almost being the tool of the act himself, is no wonder to bring him back to his self-critical mood and which makes him go back to being alone again, especially when he does not want to trouble people. As well as is it very odd of fans to critisize a new characteristic in a story of known characters because they are supposed to be people and can grow which means they can also learn and make new experience, react in a way they wouldn't have done before and make it a new part of their character. Yuna also finally had a chance to live her life. Final Fantasy X-2 with absolutely the same characteristics of Yuna would have been odd. Surely she could have been more reserved at first and then start being more enthusiastic but hey - she had the chance to live a life she didn't actually dream of for 10 years. That is a good reason to change and try out something new. Same thing goes for characteristics that apparently never were a part of a character before but depending on the character very well could be foundl, And even if not - we learn every second.