In 1998, Kasai made his debut in a Big Japan Pro Wrestling (BJW) event in Osaka during the Deathmatch Wrestling boom of the late 90s. As BJW established a working relationship with American promotion Combat Zone Wrestling (CZW) at the turn of the millennium, Kasai joined the stable "CZW Warriors" (CZW Army) and feuded against the "BJW Warriors". Kasai formed the stable "Big Dealz" in 2001, with John Zandig, Nick Mondo, Wifebeater, Z-Barr and Trent Acid.[2]
Kasai has participated five times in an extremely bloody "Razor Deathmatch"; a hardcore match where boards are fitted with many razor blades. The first of these deathmatches was held in 2005 with Jaki Numazawa, in which Jun took a powerbomb onto the razor bladed board. In the second one, held in 2008, Kasai's match was against the American wrestler Masada. On November 20, 2009, he defeated Ryuji Ito in another razor match after a double underhook facebuster on a cactus.
The fourth time was on April 5, 2010 where Kasai competed in a Barbwire Treaty Death Match that pitted Team CZW (Kasai, D. J. Hyde and Nick Gage) against Team BJW (Jaki Numazawa, Isami Kodaka and Masashi Takeda) in which a razor blade board was used in several bumps. On June 21, 2010 Kasai competed against the Necro Butcher in a Hardcore match for the Freedoms promotion; the razor blade board came into play mid way through the match.
In 2012, Kasai teamed regularly with American pro-wrestler/rapper One Man Kru. Together, they defeated the team of The Great Sasuke and Kesen Numajiro in a fluorescent light tubes death match at the September 9, 2012 Freedoms event in Iwate, Japan.
Kasai is married to his wife Michiyo; the couple have a daughter together.[10] Kasai also has a son from a previous relationship.
He was born in Hokkaido and now lives in the mainland, but claims on his official website that he is from "Hiladelhia, America" as a joke;[11] he has described Philadelphia as his second homeland. While Kasai wrestled in the United States, he was a lodger in Matthew Prince, better known as the wrestler Wifebeater's house. He has described Prince as a real madman, unlike many deathmatch wrestlers who are in fact gentlemen outside the ring.[12]