Alright, here goes.
Terenia: Terenia was born human who became an Andalite
nothlit. Unaccepted in Andalite society, she was instead taken to be experimented on in an attempt to cure
nothlitism. Eventually the scientists succeeded, but their breakthrough was short lived. Terenia managed to escape the lab (which was aboard a Dome Ship) and escape into space. A series of events followed in which Terenia killed her brother from Earth at the bidding of Visser Three andpunished herself by becoming re-
nothlitized as a Yeerk. When the Andalites find out they again find an alternative use for her, using her Yeerk form as a tool in espionage. Any time a former host is captured in battle she is forced to enter them, make them become involuntary, extract useful information, and then leave. Eventually she gets fed up, steals an Andalite for a host, and runs back to Visser Three with the intent of saving her one surviving family member, her twin sister Tamora.
This fails when Tamora is kidnapped by Ossanlin & Co. and Terenia decides that there is no saving her sister (who is a voluntary Controller). Instead, Terenia kills Tamora, granting her the ultimate "freedom".
Tamora (
deceased): Tamora entered the bar when Ossanlin, Myitt, Claxter and Aliciana kidnapped her and her Yeerk from the Blade Ship. Her Yeerk, Zorish, was starved out and Tamora, who was voluntary, was left to fend for herself. She tried to seek revenge, putting hits out on Myitt and Terenia both through a bounty hunter. When that failed she tried making Myitt her new Yeerk. Myitt wouldn't have it and turned Tamora over to Ossanlin. Ossanlin in turn gave Tamora to Terenia, who killed her.
Efaen: Efaen is a female Andalite who had just finished her
aristh training when Terenia captured her and forced her to become an involuntary host. For a long time the two fought bitterly for control, but over time Efaen grew to grudgingly accept her fate, if only because it allowed her to be close to the Yeerks and plot against them. Efaen was temporarily freed, but chose to accept Terenia back because she fears that as a female Andalite in the military (and one who Visser Three would likely target) she will not be allowed to fight the Yeerks properly. Instead, she chose the possibility of espionage with Terenia controlling them. This is not to say the two get along. They tolerate one another at best, and usually seek to make the other miserable. Efaen has a rather old-fashioned view of Andalites, believing that they are superior and all others inferior. The Yeerks, especially, are incapable of anything but evil.
Trey: Trey comes from a universe that exists twenty years after the Yeerks lost the war for Earth. He is the son of two Yeerks who were forced to become
nothlits when the war was lost. As a 'grub' he grew up a second-class citizen, deprived of many basic rights. He left Earth with his father, with plans of beginning a rebellion. The plans went astray when their ship crashed on the planetoid housing the bar for unknown reasons. Trey's father died due to injuries sustained in the crash.
That's the brief version. If you want to read Terenia, Tamora and Efaen's story in more depth I am posting it under the Fanfiction section here:
Playing War: The Traitor.
