A good starting point for Kairi's character development is to not have everything revolve around Sora.
If a character is centred in promotional material, that's symbolicly represents them being the central character.
Take this image for example. Ventus and Marluxia/Lauriam are the more recognised characters but it's Ephemer who is in the centre... but why? Well, it's because he's arguably the central character of the X saga if we ignore the player character.
Oh, there most definitely is sexism in play. Take Birth by Sleep, Aqua is the most removed character from the conflicts of the game. Terra is the character that Master Xehanort interacts with the most, Ventus is the character that Vanitas interacts with the most whilst Aqua is just an inconvienience for Vanitas and never speaks to Master Xehanort.
It was only after the popularity of Aqua compared to Terra and Ventus that Nomura retroactively made Aqua the most important member of the Wayfinder Trio.