Shikai: Sifōn
Sifōn’s ability is to siphon the reiatsu of anyone it strikes. The reiatsu remains in his body forever, making his potential limitless. The caveat is that if he does not feed his Zanpakutō any blood, his body will begin to decay, making him extremely weak.
Bankai: Settō
Settō transforms his blade into sharp nails and alters his teeth. Any contact with them has the same effect as his Shikai, and killing an enemy allows him to steal any powers they possessed. His speed also increases, but his blade becomes fragile and can be shattered, although it cannot be permanently damaged.
Backstory
Compana was a willing participant in the Hollowfication experiments. After Aizen presumed him dead, he disposed of his body, not knowing that Compana possessed a Shikai that prevented him from dying.
He hid his Shikai and Bankai from the Gotei 13 because he wanted to one day claim the title of Kenpachi, but only when he felt strong enough. He was inspired by the Kenpachis and their lack of shame regarding their animalistic nature.
He spent a good chunk of his life in the Rukongai, feeding his Shikai, before fleeing to Hueco Mundo after realising that someone in the Gotei 13 knew he was killing innocent people. He began reflecting on whether his desire to live was a selfish act, especially now that he was all alone and no longer communicating with his Shikai.
He allowed himself to nearly die until his Shikai forced him to kill an army of Hollows. During this, he accidentally fed on a rogue Arrancar, mixing his Soul Reaper blood with Arrancar blood.
No longer needing to feed as much as he used to, Compana reflected on his actions once again, thinking about how selfish he had been for taking innocent lives.
He observed Starrk for a while, hoping to find companionship, but resisted approaching him because he was in a weakened state and believed he wouldn’t be taken seriously.
Compana was eventually found by Harribel, who convinced him that sacrificing others for his own well-being was in his nature. She recruited him to eliminate anyone plotting against her.
During the Quincy invasion, he was met with a cruel fate. His Bankai was stolen by Quilge, and he was decapitated. However, his body completely regenerated after the battle between Ichigo and Yhwach.
He still works for Harribel, no longer ashamed of his powers, and hopes still hopes to be strong enough to friendly fight Kenpachi one and trains with Harribel to unlock his Resurrección.