Chiteiko(地底湖) - "YOSHIKO ver.1.01" (CD+letter)

I received a CD titled "YOSHIKO ver.1.01", an EP by a band called Chiteiko, and a letter... Letter?
What is Japanese Abyssal Core? I think it is a progressive young indie rock band with a drummer, Vo/Ba, and two guitarists, but the details are still unknown. There are some Japanese-style guitar riffs that are reminiscent of Human Chair or Chugakusei Coffin, and I think there are elements that will resonate with listeners with a metal brain.
Although the sound quality and the performance itself are rough, the band's worldview is well expressed. The direction of expression seems to be divided into riffs and songs, and I personally did not find the song-oriented expression to be very appealing, but I can feel their worldview in the songs that are pushed by riffs and performances. The soundtrack also comes with a "letter". What does that mean?

tracklist:
1. 村の娘
2. よし子落雷
3. 星の娘
4. 陽炎