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. In 2006 he won the first-ever IWA East Coast Masters of Pain after defeating Toby Klein,
Mad Man Pondo and
J. C. Bailey. 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. On January 3, 2021, Kasai defeated Yoshitatsu to win the
All Japan Pro Wrestling's
Gaora TV Championship. On July 4, 2023 Kasai made his
New Japan Pro Wrestling debut at
Independence Day in a winning effort in the main event teaming with
El Desperado against
Jon Moxley and
Homicide. ==Personal life==