He serves as main antagonist in The Warning.
He ans Esplin 9466 born from the same grub in an artificial Yeerk pool on board a captured Andalite ship shortly after the Yeerk exodus from their home world. It is usual for one twin to become a success and gain power and influence, while the other - designated "Lesser" - is left with nothing though the rare case shows that the superior one and the lesser still able to work together nevertheless. An instance that unfortunately not last long was between Esplin 9466 with his lesser twin. Whilst the superior Esplin whom eventually became the Visser Three, the Lesser ended up left infesting a lowly computer programmer named Joe Bob Fenestre during their invasion together on Earth along an army of Controllers.
Unsatisfied with his lesser status as the infiltrators of the Yeerk Empire, Esplin 9466 Lesser decided to turn his misfortune around. Using Fenestre's knowledge of human businesses, and his own knowledge of advanced computer systems, he created Web Access America, which quickly became the world's leading Internet Service Provider. The success of the company made Fenestre a billionaire and household name, and gave Esplin a vast information network - not only the names and addresses of WAA's subscribers, but more personal information gleaned from e-mails and chat rooms. Esplin quickly gains influence in human society, making him an asset to the Yeerk invasion.
Unfortunately, this costed him his trust and relationship with his brother, whom has became the Visser Three. His popularity, arrogance, and power hunger nature prompt Visser Three to declared him a traitor, forcing him into hiding. Knowing that his twin's host had the ability to morph into animals of all sorts, he equipped his human host's mansion with weapons, ranging from Rottweliers and guards with submachine guns to insecticides and electrical force fields. His guards had instructions to shoot anything that attempted to come close to the house. Though Visser Three knew this, he hestitates to send his underlings to attack as he knew that Lesser still survive without need of Kandrona Ray radiation that normally required by Yeerk to survive.
Events in The Warning
The Animorphs became suspicious of Joe Bob Fenestre when they infiltrated the Web Access America offices, looking for the names and addresses of users in a chat room whom they believed were in danger of Yeerk infestation. They discovered that Fenestre had been a participant in the same chat room, with the username "Fitey777", and decided to investigate Fenestre's mansion.
Although the mansion's security was difficult to break through (with Rachel and Ax captured in process), they were eventually able to gain access and meet with Fenestre, whom revealed to be a Controller whom Yeerk inside him revealed to be Esplin 9466 Lesser, Visser Three's twin brother. Esplin explained to the Animorphs that the only reason that he was still alive was due to a technique he had discovered that let him substitute a powder made from the bodies of Yeerk for Kandrona rays, sustaining his life without needing to visit the Yeerk Pool (as he did not have access to it like other Yeerks) but forcing him to become a cannibal. He presumed that Visser Three allowed him to live only because he desired Esplin's technique; but, as he explains, the technique would never catch on in Yeerk society.
After Esplin had told his story, Cassie, horrified by his cold-blooded attitude towards humans and Yeerks, demanded that Jake kill Esplin. Jake, however, had already agreed not to harm him, but as a compromise, adds a condition: Esplin may never leave his mansion again. Esplin agrees; however, a few days afterwards, Fenestre's house burns down under mysterious circumstances. It is implied, but never stated, that Jake or Cassie or Visser Three himself may have done this (Jake's narration stated that there are many names listed in the potential suspects for the burning and he or Cassie was on the list). Nobody died in the fire, but Esplin's fate was never discovered.
He is an evil genius Yeerk whom arrogant, power hungry, and bitter. He had no qualm in killing his fellow Yeerk and their hosts simply to survive by turned their bodies into Kandrona powders that consumed through his host's digestive system. He held terrible grudge against Visser Three, because his attempt to turn the tide of his luck turned out threatened his position.