I agree. I was also disappointed to find out they wouldn't be together at the end. but at the same time this is more realistic. i think she only thought of him as a very dear friend that saved her from her loneliness and that hellish situation. like in the case were she slept over, and his mom said if she was getting in the way, Kayo was kinda shocked that she even suggested that and instantly denied it. I think the relationship would progress into lovers if they would be older. but since they were just kids, he remains an important (probably also first) friend. and that is how she treats him when they get older. It is obvious that it those 15 years she would find love in somebody that was actually there, beside her. but I could be overthinking this, since I know how it ended ╮(￣▽￣)╭ I am just glad that they choose someone that was a part of story, and not someone totally new. and that in the end both of them are happy, even though they are not together(▰˘◡˘▰)
Seriously, what the hell were the writers thinking? They built up Sotaru and Kayo's relationship through the entire story, then at the very end throw it away all away. They basically just slapped all the readers in the face.