Esplin 9466, Visser Three, or later, Visser One is the main villain in the Animorphs series. A ruthless Yeerk, he was the first of his kind to infest an Andalite host body. With the body, he was able to tap into Andalite technology, for example taking on different forms. Esplin had a twin brother named Esplin 9466 Lesser, whose fate is unknown. In the series he tried to cause an invasion on Earth. He is also the nemesis of the Animorphs. He was also known for making up excuses to execute people. In book #45, he was appointed to Viseer One after the failure of the original. At the end, he accepts his defeat saying that "Any punishment that you inflict, would be nothing compared to what the Council of Thirteen would do." His ultimate fate was being incarcerated in prison deprived of sight, and without a host body. He also appeared in the television series.
Esplin 9466 was a Yeerk who hailed from the Yeerk Homeworld. Esplin had a twin brother named Esplin 4966 Lesser. At some point in his life, he joined the Yeerk military with the intent of conquering the galaxy under the Council of Thirteen, supreme commanders of the Yeerks. Esplin participated in many battles during the Andalite-Yeerk War and during an attack on the Andalite Homeworld, Esplin personally infested and gained control of an Andalite war hero named Alloran Corrass, though the Yeerks failed to conquer the planet. With his Andalite host, Esplin became the only Controller(a being controlled by a Yeerk) with the Andalite's ability to shapeshift or morph into any creature, sentient or non-sentient. Esplin became very powerful and rose quickly through the ranks and eventually became known as Visser Three. Under his command, Visser Three assisted the Yeerks in infesting many races, such as the peaceful reptillian Hork-Bajir and the violent insectoid Taxxons. Later, Esplin was assigned to conquer Earth, the homeworld of the peaceful Humans, which he agreed to carry out. However thoughout his military career, Visser Three confronted an andalite named Elfangor who, like all Andalites, wanted to stop the Yeerks from infesting the galaxy. Visser Three received word that Elfangor had been making his way towards Earth and eventually tracked him down. Visser Three attacked in his Blade Ship and a fleet of Bug Fighters, and confronted an Andalite military force. Eventually, Visser Three and his forces shot down both the Dome Ship and Elfangor's spacecraft. With the Andalites dealt with, the Visser led his forces down to Earth, where they immediately began their invasion.
Visser Three discovered that Elfangor survived the crash and immediately went after him. Visser Three eventually tracked down Elfangor to an abandoned construction site and landed to confront him. Elfangor quickly fought and defeated Esplin's Hork-Bajir forces, but the Visser morphed into a hideous bulky creature and swallowed Elfangor whole, killing him. After that, Visser Three's men destroyed Elfangor's ship, but Esplin sensed that there were "Andalite Bandits"(the Animorphs, five humans with the morphing ability) on Earth to stop him and ordered his troops to find them and eliminate them. Later, Visser Three and his men began infesting Humans and constructed a Yeerk Pool underground for the Yeerks to feed and to cover up their operations from the Human government. Esplin also turned a man named Hedrick Chapman into a Controller and fellow Yeerk Iniss 226 infested Chapman and he founded a group called The Sharing to acquire new hosts for Yeerks. A teenage Human namd Tom was also infested. Later, a policeman controller kidnapped a teenage girl named Cassie(one of the Animorphs) and held her hostage in the Yeerk Pool. Later, the other four Animorphs led by Jake, Tom's younger brother, attacked the Yeerk Pool and rescued Cassie and many other captives. They also killed many controllers, but Visser Three and his controllers outnumbered them. Jake fought against the Visser himself in tiger morph while the Visseer transformed into a fire skinned creature that could shoot fireballs. Despite his powerful strength in morph, Jake defeated Esplin and bit into his chest, injuring him. Esplin shot two fireballs to kill Jake, but Jake repelled them back at the Visser and defeated him for the first time, leaving him critically injured. Jake quickly killed the policeman controller and escaped with the rest of the Animorphs, but not before the wounded Visser Three vowed to kill them and have revenge.
Visser Three eventually recovered from his injuries and made plans to infest Melissa Chapman, Hedrick's daughter. He contacted Chapman through holographic technology and asked him to infest Melissa, but while he was talking, Visser Three notices Chapman's cat spying on them, and ordered Chapman to kill the cat because it might be one of the Animorphs, unaware that it was Rachel, one of the Animorphs in disguise. Chapman allowed Rachel to escape unharmed. Later, Visser Three caught the cat spying on Chapman discussing matters with him again. Visser Three soon realized that the cat was one of the Animorphs and ordered Chapman to bring the cat to him and Melissa so she could be infested. Chapman briefly rebelled against Iniss, which opted Iniss to leave Melissa and bring Rachel. Visser Three met with Chapman at the abandoned construction site where Chapman revealed the only reason he became a controller was to protect Melissa. As his Taxxon troops carried Rachel into the Blade Ship, the other Animorphs arrived to save Rachel and Jake, who was disguised as a flea, morphed to fight Visser Three again. Jake yet again overpowered Esplin and Tobias, one of the Animorphs disguised as a red-tailed hawk, grabbed the cage containing Rachel and flew off with the rest of the Animorphs, but not before the Visser vowed revenged again and ran off into the nearby woods in a fit of rage having been foiled by the Animorphs a second time.