The author will give a brief summary:
Before Yujin's soul transmigrated into the Bleach world, Fūjin Taichi had already refined an unnamed Asauchi into his own Zanpakutō, named Kyūjūkyū (Ninety-Nine), whose sword spirit was Yone.
After Yujin and Fūjin Taichi merged, using a new soul state, they refined another Asauchi that Yujin had picked up into a new Zanpakutō—Ichi (One), whose spirit is Wind God · Yasuo.
During the Konso (Soul Burial) training, "Yujin" lost the Zanpakutō Kyūjūkyū, but its Zanpakutō spirit still remained within his body. Because it was abandoned by its "master," it degenerated into a Hollow and took the name Wind Demon · Yone.
(Think of the Muramasa Zanpakutō Rebellion arc in the anime for reference).
Because Yujin had continuously been using the power of Wind Demon · Yone, Zanpakutō Kyūjūkyū was easily destroyed by him, and the so-called "Shinigami power" was reclaimed.
In Chapter 45, after the protagonist's Shinigami power was taken away and he was affected by Wind Demon Yone's Hollow power, he began to undergo Hollowfication.
At the brink of life and death, he awakened the true power of Zanpakutō Ichi within his body and returned to being a Shinigami.
(Refer to Ichigo Kurosaki's storyline where his Saketsu and Hakusui were destroyed by Byakuya Kuchiki and he re-forged his Shinigami powers.)
Additionally, due to subconscious influence from "Yujin," Fūjin Taichi had long harbored fear toward Aizen, which is why he was never able to defeat Wind Demon Yone.
Now, Fūjin Taichi has reclaimed his true strength, completely cast aside fear, calmly accepted his identity as a Shinigami, and has finally begun to confront Aizen head-on.
Notes
PS:
Both Kyūjūkyū and Ichi are powers belonging to Fūjin Taichi, and therefore can both be used by the protagonist.
(Refer to how Ichigo Kurosaki possesses Quincy, Shinigami, and Hollow powers simultaneously, yet all fundamentally belong to himself.)
PSS:
Fūjin Taichi's career progression:
Shin'ō Academy Special Advanced Class → 13th Division Unseated Officer → 13th Division 20th Seat Candidate, 15th in strength (promoted after saving Shiba Miyako) → 1st Division Vice-Captain (current)
Graduation Statistics
Zanjutsu (Swordsmanship: First-Class
Shunpo: Elite (Approaching the level of "Utsusemi")
Kidō: First-Class
Hakuda (Hand-to-Hand): Average
Zanpakutō Details
Zanpakutō ① – Ichi
Abilities: Wind manipulation + spatial control
Shikai: "Face the wind—Hasagi!"
Bankai:
"With the sword in my heart, I listen to the wind's hum—
Bankai: Blade-Master Wind-God—Twenty-Four Celestial Kings!"
Zanpakutō ② – Kyūjūkyū
Abilities: Flame manipulation
Constant-release type Zanpakutō
Can be freely mixed and used together with Ichi
Bankai (Hollowfied State):
"Death is like the wind, always by my side—
Bankai · Blazing Flame Demon · Twenty-Sixfold Purgatory!"
Zanpakutō ③
Resurrección: Unknown
Segunda Etapa: Unknown
(Note: Fūjin Taichi's Zanpakutō is a dual-attribute weapon of wind + fire.
After the reborn Fūjin Taichi seized the wind attribute, it became the Wind-Controlling Blade, which is why Wind Demon Yone always claims that the protagonist's power was stolen.
Additionally, the reborn Fūjin Taichi's Zanpakutō now possesses space + wind attributes, so the techniques it releases have both wind-control and spatial abilities.)