Complex sentence: Shirai left The High-Lows in 2003, and eventually joined with a third former member of The Blue Hearts, Tetsuya Kajiwara, and formed The Big Hip.
Split into simpler sentences: Shirai left The High-Lows in 2003. While embarking on a solo career, he also joined with a third former member of The Blue Hearts, Tetsuya Kajiwara, to form another band, The Big Hip.