I think it's Kame. That "aishiteru" sounds a lot like how some lines in "aishiteiru kara" are sung, for example. Then, Koki's wispers (for example the rap in Silence, or the "kocchi koi yo" at the beginning of MUWII) are different because Koki's voice is more raspy than Kame's.
Of course this is what I think :)