Taiji Ishimori made his debut as Mexico Arena Coliseo as a graduate of the prestigious Toryumon school in 2002. After returning from Mexico, he was mainly active in the Dragon Gate promotion. He moved to Pro-wrestling Noah in 2006 and defended the GHC Junior Heavyweight championship a record 10 consecutive times.

In March 2018, he left Pro-Wrestling Noah, and was recruited by BULLET CLUB.Whilst masked under the guise of the Bone Soldier, he attacked Will Ospreay, who had just defended the IWGP Junior Heavyweight Championship against KUSHIDA, at “Wrestling Dontaku 2018” held at the Fukuoka International Center on May 4th. Ishimori would declare himself, the Bone Soldier, and the entire NJPW Junior Heavyweight Division, ‘Reborn’.

At Wrestle Kingdom 13, he defeated KUSHIDA to become the IWGP Junior Heavyweight Champion for the first time in his career. After a successful defence against Ryusuke Taguchi at New Beginning a month later, Ishimori would call out the legendary Jyushin Thunder Liger, saying that he could only truly make the division reborn if he defeated its greatest figure. Their clash at the 47th Anniversary Event was a memorable one, but a month later, in Madison Square Garden, Ishimori lost the gold in a three way match to Dragon Lee.

Ishimori would bounce back with a strong Best of the Super Juniors, though Shingo Takagi’s dominance in his block saw Ishimori come second. Ishimori would nonetheless form a solid team with El Phantasmo by the end of the tour, and in June 2019 the two would defeat Roppongi 3K to become IWGp Junior Tag Team Champions.