Some people have expressed some interest in Kei's growth being a more involved element of the story. From the story standpoint his growth was selected as something that wouldn't be all that intrusive to the plot but still enough that allows him to be part of the setting. Though Kei is referred to as the main character, the real focus of the story line is the datable girl of your choosing, with the plot largely focused around their growth along with the perks and challenges that come with it.
The idea to give him a different growth is something that would have to be explored in a fan-mod, which is possible if someone has the coding skills and understanding of the characters. This is because unfortunately, changing his growth would impact the sexual chemistry with his girlfriend as well as his relation to his sister and her growth in the MC and RM storylines, in addition to all of the art assets that depict his appearance with his normal growth.