In Rune’s we mention it while on the roof with him. Also in Mikko’s route he mentions how ‘non linear’ his life feels. I’m guessing he’s saying that he feels as though he’s defied any kind of predestination in his life, but I could be mistaken.
Right, so, in Rune's roof scene Arvo is thinking about non-linearity in a whimsical way that isn't supposed to make much sense, I think - he's just experiencing a case of deja-vu, feeling the same way he did long time ago, that sense of inspired hope for the future and closeness to someone he mutually appreciates, as if the life looped back on itself and the same moment happened twice.
And... I don't quite remember what Mikko said about non-linearity. Could you remind me when he said it so I can double-check?
It was Arvo both times. He just said kinda the same thing to Mikko as he did to Rune. You might be right, he could be talking about the purpose and meaning of memories in our lives; timelines folding in on themselves as our minds bring up the events of moments passed in the present, affecting our current actions and thoughts, though the moments themselves are gone forever. That's quite cool to think about actually.