The Changeling who assumed the form of Andrew Ellis was an outcast Founder who had been banished from the Great Link. He kept the body of the real Ellis in stasis, using a scalpel to give to illusion he had been attacked by an animal. Ellis was his primary guise during that time, also assuming the form of a Bajoran named Arak Katal. His colleague Crell Moset had become accustomed to referring to the Changeling as Ellis. After the Cardassian Union joined forces with the Dominion, the Founders chose not to target family members of known Maquis members, but the Ellis Changeling in the guise of Arak Katal disclosed the location of Tevlik’s Moonbase to the Cardassians. As punishment for defying the Great Link, the Ellis Changeling was stripped of his shapeshifting abilities.
At some time during the Dominion War, the Ellis Changeling liberated the Cardassian scientist Crell Moset from Federation custody. Moset provided him with injections that partially restored his abilities, and he could only assume the forms of humanoid males.
The Ellis Changeling later brought the real Ellis’s body out of stasis as part of a ploy to assume the form of Captain Chakotay, after turning the real Chakotay and his sister Sekaya over to Moset. After the away mission to Loran II, Lieutenant Harry Kim and Dr. Jarem Kaz became increasingly suspicious of Chakotay. The Changeling eventually revealed himself and escaped in the guise of Kim with Kaz as a hostage.
Apprised of the situation, Lieutenant Commander Tom Paris took the Delta Flyer II to Voyager to assume command and led an away mission to rescue the missing officers. Moset was captured, but the Ellis Changeling escaped. (VOY novel: Spirit Walk)