Acid was a Japanese rock band originally created by Hideki after Siam Shade disbanded.
With the end of Siam Shade, Hideki didn't want people to always compare the two bands, which is why he chose to change his position to behind the keyboard, before later leaving Acid completely in June 2005.
Furthermore, he stated after his departure that Acid is his pride and that he would like to see them perform live at Tokyo Dome one day.
[1] In the beginning, Hideki had a huge influence on Acid and they played a mix of alternative metal and emo with emphasis placed on his and Junpei's twin vocals.
After he left the band in 2005, their songs became less focused on vocals and turned a bit more pop-ish, but still kept their "heavy" sound due to having three guitarists.