Just thought I would throw in my two cents here, apologies if this is
First of all, you can't expect every character in a series to share an equal level of power to the protagonist. Especially when it comes to fledgling Keyblade wielders (Kairi and Lea, specifically) they are more likely to "job" to those who are way more experienced in combat (all of the Xehanorts). Expecting Kairi to have gone out and made tremendous strides would seem overly convenient, and you are forgetting that she did do things in the story to make her more than just damsel in distress like in Kingdom Hearts II. Not only did she participate in all of the battles leading up to the battle with her and Lea involved, but she trained the entire time with the sole intent to do as well as possible. Though I wish she had a different story arc, to claim that they didn't treat her fairly is pure wishful thinking. Killing Kairi was also necessary to motivate Sora to actually fight Xehanort to give him his final key, because Sora could have just refused to clash. It was not an unnecessary kill, in my eyes, and it definitely made up for the rather convenient sequences leading up to the Xehanort clash.
Aqua shares a similar issue, where she finally escapes the Realm of Darkness that polluted her mind for years, and escaped an even more powerful darkness that possessed her. It's been seen several times that recovering from the influence of darkness takes time, the most recent and potent example being Sora losing all his powers at the end of Dream Drop Distance. Though it is weird how she just stands there against Terranort in Keyblade Graveyard, you could argue that, like Sora, her hopes were being crushed right in front of her during that scene.
Namine literally had no reason to exist prior to Even making more replicas, so to say that she is unfairly treated is, once again, expecting too much.
Xion is more understandable, though you have to remember the hesitation she will definitely have when being ordered to kill her best friend by someone as powerful as Xemnas.
I think it's ridiculous to say that they are treated unfairly given the circumstances these characters are presented with. If anything, you could argue that every character but Sora is unfairly treated in Kingdom Hearts III, since they all seem to "job" in their own special way. I understand the desire for female characters to be more adequately represented in the games industry, but I don't think that this is the best game to critique that aspect on. There are plenty of other issues with the narrative of Kingdom Hearts III that can be dissected.