Yammy used to be the 10th strongest Espada in Aizen's army but it is revealed that he was actually number Zero.
Yammy Rialgo is one of the many arrancar in Sosuke Aizen's army, and one of the few Espada in which he holds the rank of deecima (spanish for the number ten). Yammy makes his first official appearance during the arrancar arc in which him and Ulquiorra seemingly fall from the sky. Their decent causes a large crater to appear in Karakura Town, something which attracts the attention of many of the citizens nearby. This is the first major appearance of the Espada though they are not known as such until later on. During their arrival, Ichigo and Soul Society become aware of their sudden arrival. Yammy comments that he had visited the human world a few times with his mask on and that it is still boring. He also comments on the lack of spiritual energy. This prompts an annoyed reaction from Ulquiorra in which Yammy apologizes, thus showing just who is in charge. All the while the two are surrounded by people who Yammy thinks is staring at him despite them seeing nothing. Annoyed Yammy sucks out their souls. Only Tatsuki is able to survive the attack from Yammy.
Thinking because Tatsuki survived his gonzui, that she was a strong spirit and the person that him and Ulquiorra had been sent to scout. Ulquiorra tells him differently and that she is just trash because she was close to fainting just being near Yammy. Yammy then decides to kill her; however, he is stopped by the sudden appearance of Chad and Orihime. Chad engages Yammy in a fight which the Espada makes short work of, as he crushes Chad's arm with little effort. Yammy then has a fight with Orihime after she returned to save Chad. In the fight, she reveals her unique abilities but is powerless against Yammy's strength who crushes Tsubaki. Just as Yammy is about to kill her, Ichigo shows up and saves Orihime. He becomes aware that the person they had come to scout was Ichigo. Yammy immediately attacks but his attack was easily stopped by Ichigo's bankai. Yammy then has his arm cut off by Ichigo. After this Ichigo's hollow starts to show, Ichigo is forced to back up. This gives Yammy an opening and he proceeds to beat Ichigo and Orihime.
Just before Yammy can do any more damage to Ichigo, Kisuke Urahara and Yoruichi Shihoin save him to put a stop to Yammy's attack. Overconfident Yammy attacks the pair but he is easily tossed aside by Yoruichi. Deciding to attack again, Yammy is yet again defeated by Yoruichi who knocks him down. Getting angrier by being handled so easily by Yoruichi, he again attacks her but only this time he attacks with a cero, but the attack is stopped by Kisuke who uses Benihime to launch an attack. Ulquiorra then steps in and stops it. Yammy is then hit by Ulquiorra and told that he stands no chance against either Kisuke and Yoruichi in his current state. The two Arrancar then retreat back to Hueco Mundo. Back at Hueco Mundo, Yammy has his arm reattached and tests it out by attacking the girl who had reattached it. Yammy along with Grimmjow, Wonderweiss and Luppi are sent back to the living world to launch an offensive. Yammy engages Toshiro in a fight but Luppi soon takes over and fights, leaving Yammy with any opponent. Yammy is then shown watching over Wonderweiss who is playing with the dragonflies. Later on during the fight between Luppi, shinigami Kisuke arrives. Yammy then attacks Kisuke, wanting revenge from their last encounter. The fight is one which Yammy again seems to be losing. He is then taking back to Hueco Mundo.
Hueco Mundo arc
Yammy is later shown to be apart of the meeting between the ten Espada and their leader Aizen along with Tosen and Ichimaru. Along with a few of the others he complains at the arrival of Ichigo, Chad and Ishida. Yammy later learns of the death of a fellow espada. The number Nine Aaroniero but he simply scoffs at the news and doesn't seem all that affected by his comrades death.
Here Come Zero
Yammy suddenly appears during Ichigo and Ulquiorra's fight. He want to help Ulquiorra in the fight.
Ulquiorra refuses and tell Yammy to go back to sleep. He got mad and kill menoly and loly then he ask to kill Orihime. Uryu arrives and shoots him in the shoulder, knocking Yammy down. Yammy got back up and try to attack but a landmine explodes under his feet. He fall
through every floor of Las Noches. Mad at Uryu for making him fall and Ulquiorra for dying. He then releases his Zanpakuto in a rage. His number change to a 0. That means he is the strongest Espada.
Yammy is unique among the Espada, because he changes his rank when releasing his sword. He is the tenth Espada, but when he releases he becomes Espada number Zero, the strongest Espada. Like all Espada Yammy posseses the standard hollow powers such as Sonido, Hierro, Cero etc.
Superhuman Strength: Yammy has incredible strength, able to kill or maim people just by using his bare hands
High Spiritual Pressure: As the 10th Espada yammy has higher spiritual power than most Arrancars, after releasing his spiritual power is off the charts
Hierro: Like all Arrancar, Yammy posseses the ability to harden his skin
Ira (spanish for "Anger," Japanese for "Angry Beast")