This is... uncomfortable. After doing some math, I determined that Eichi must have been at least 14 years old when Mitsuki was 10. That means a middle schooler went and kissed a fourth grader and proclaimed his love for her.
That's really, really disturbing in real life. I mean, where I am, that would be the same as an eighth grader (someone going to enter high school) being in love with an elementary schooler. I don't like this romance.