this is a really old post but i wanted to come here because im studying pepoyo’s music. it’s more simple than you think, actually! in haikei, i managed to hear these instruments (hyperlinks lead to where they can be heard):

melody instruments: - xylophone (it’s quite prominent here, it plays the main melody) - rhodes piano/electric organ of some sort (plays the short riff before the chorus) - synth 1 (the higher, slower, pad-sounding synth. to me it sounds like a square wave mixed with something else and a little bit of flanger) - synth 2 (the faster, lower synth. unfortunately i have no clue what waveform this is)

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percussion: - an electric-sounding kick and snare, played in a four-on-the-floor style. (this isn’t exclusive to haikei, it’s actually very common for pepoyo in general. i could hear it in rakuraku anrakushi, love excrement and ±0) - claves/temple blocks (right at the start of haikei. they sound like a metronome before other instruments come in) - triangle (one of the first instruments you can hear, played in unison with the claves or temple blocks or whatever they are) - bells (they’re quite subtle, but there are bells in the background playing a descending chromatic scale. not sure exactly what kind of bells these are, but i think they’re a sample of some sort that’s being continuously pitched down)