Yes, you can stay only friends in all of the routes ~
I think you might have noticed, but depending on your answer when the choices are presented during the first half of the chosen route, you got either a yellow heart or a heart with a color related to the character's route (eg : blue heart for Ikki, red heart for Toshio, etc). If you want to stay friends with the character and don't want to go the romantic way, you'll have to gather more yellow hearts than other coloured ones. And if you want the romantic path, then you'll need to gather more of the other coloured hearts (^ O^)/
Unfortunately, there is only two endings (the Friend ending and the Sweet ending) for each characters, which is I admit not a lot (and the story itself is pretty linear as well)...
The Friend ending is the one where you only stay friend and has really slight differences with the Sweet ending, while the Sweet ending is the continuity of the Friend ending with a bit more details on the characters backstories and a small romantic aspect + cute moments between the MC and the LI ✦
That said, thank you so much for loving Ikki's story so far ! It really makes me happy and I hope you will like the other ones too ❤