What some say is that harmony, song structure, rhythm, can be analysed rationally, but melody comes from the heart. Still, there are some habits that Jpop seems to have formed.
The song begins with a 4-3-2-5 with a little turnaround (1dom,7dom,b7dom, 6dom(substitutional dominant of 4))then ending on the 1 on the second pass. The melody for the first bar can be thought of as two halfs, the first going between the 9th and 3rd notes of the IV chord, and the second going from the fifth of the IV chord to the half step below and returning. The first half repeats exactly on bar 2, the notes this time making up the 3rd and 11th of the chord. This is called "call and response" melody writing, where you have a statement, and then respond to it, in this case the first and second half of each bar acts as a call and response, as well as the two bars in relation to eachother as the ending of bar 2 is different from the ending of bar 1, making them distinct statements. The first half of each bar is made recognisable and makes an impression on us by being repeated, lending to it's catchiness.
As for the strength of a melody, generally strength is understood in two ways. The beat the note is played on, and the notes relation to the chord. Notes played on the beat are considered strong, and notes from the chord are considered strong. Then on the inverse, notes on the off beat and notes not in the chord are considered weaker. What is on the beat?
In 4/4 timing, the time signature for most songs, the beats are counted 1, 2, 3, 4. 1 and 3 are the "on beats" 2 and 4 the "off beats" Further, these gaps can be divided multiple times, creating 8th notes, 16th notes, so on. 1 *and* 2 *and* 3 *and* 4 *and*... Notes on these subdivisions get progressively "weaker" and this is where people put passing tones, notes not in the scale that can be used to move more fluidly, because they're on weak beats the fact that they're not in the scale is made less jarring, as they take up less importance in our perception.
While notes not in the chord are weaker than notes in the chord, the only truly "weak notes" are either those not in the scale, or notes in the scale that still clash with the chord. For instance, playing the 7th scale degree over a 4 chord will create a tritone interval with the 7th of the chord, which is generally a sound that musicians will not like to sit on, and would rather pass over on a weak beat.
Repition in Jpop can't really be understated. More than "how is repition used in jpop" it may be better to ask "how is repetition not used", or how is it given enough variation to avoid being grating, but still recognisable. Essentially, small rhythmic variations, different beginnings or endings to phrases, passing tones, moving the structure of the melody up or down the keyboard are all techniques used to make it less noticable that you're hearing the same thing again and again. By small, rhythmic variations, I mean starting the melody on the off beat instead of the on beat, or changing the placement of notes to make a triplet or swing rythm.
The progression goes 2-5-1-6, 2-5-3-b3-2-5.
The same melody structure gets moved up and down the keyboard, with the only variation being the little phrases sang at the end, which I didn't mark down, cus I don't feel like it. As for the rhythm, it goes:
pu ru ru ru ru - ru rin
1 *and* 2 *and* 3 - 4 *and*.
Its very common for the last note of a melody to end on the "and" one eigth note before the next bar technically stars. Here's an example from a great akiko yano song:
This song is great because in the chorus the first chord changes on the *and* before the next bar, and it gives it a great propulsion forward, like every time you're ready for the next chord change it hits you a little bit before you expect it. Akiko Yano is just a genius anyway. Also just in case you need convincing on the chord tone thing, literally every note of the chorus is one of the seven notes of the chord that's under it, except one of them which is a nine, which may as well be though of as a chord tone. It's a chord tone of a nine chord ^_^
This is just one more testiment to the power of repetition. Listen to this and try not to let it get stuck in your head.
You: "Uhh are you going to talk about Relative Minor? You list it at the top of the page...
No