Elayne Jogan was in high school when the Yeerk invasion reached its peak. The strange mood changes in her dad—who went from an apathetic parent to someone who was highly interested in her and the culture all around him—caused her to enjoy spending time with him for the first time since her childhood. Only when the Yeerk invasion was uncovered did she realize her father's shift was not due to a change in his heart but a change in his identity.
Preferring the Yeerk in her father's head to her father, she began a Yeerk rights movement that ultimately branded her a national pest and villain. Those who opposed her became violent and, during a protest, her father and his Yeerk were both killed. Her true cause for fighting now gone, Elayne fled the planet. When she arrived at the Bar, she befriended Mar, who, despite his creepy factor, was the first person to treat her with some level of respect and romantic interest since her teenage years.
Ertoran-Sorath-Anduross (Tora) was selected—along with her classmate Serid—from among the arisths of her age to engage in a secret espionage mission on Earth during the war. Serid was ordered to morph a human and pass as a Controller, while Ertoran was ordered to morph a Yeerk and reside in Serid’s brain. Only these two Andalites and their commanding officer were aware of this distasteful mission. The more time Ertoran spent among Yeerks and in Yeerk morph, the more she sympathized with them.
Serid never suspected his partner's compassion for their enemy until one evening when he was nearing the two-hour morph limit and was forced to demorph in a quiet corner of the Yeerk Pool. He and Ertoran were spotted and captured; the Yeerks ordered Ertoran to evacuate his body so he could be interrogated, and she was put in a personal Pool to await punishment for neglecting to tell anyone she had managed to gain an Andalite host. While Serid’s memories were being analyzed, Ertoran demorphed, morphed into a mosquito, and escaped the Yeerk Pool with the intention of rescuing Serid as soon as possible. She gained a human morph to pass as a Controller, but she never discovered anything about Serid's whereabouts.
When the war ended, Ertoran was plagued with shame for her betrayals, so she cut off contact with the Andalite home world, left Earth, and joined the Yeerk rebellion. Her former commander has sinced placed a significant bounty on her head, as the revelation of her original assignment—not to mention what she has been doing since—would be a scandal.
I am planning on introducing Serid to the Bar soon.