Arcee was a teacher, dedicated to the values of information and truth. She saw to it that the younger Autobots she taught were able to see the value of facts and tell truth from lies. During the War, however, she signed up as an information courier.
That didn't go too well for her.
Arcee was an Autobot intelligence officer during the Great War. She carried crucial access codes, but was damaged in the line of duty. Ratchet repaired her and tried to take her to safety, but the two were attacked by Lockdown, who had been hired by the Decepticons to bring her in.
Held prisoner within Lockdown's ship, Arcee insisted that Ratchet use his electromagnetic pulse tool to wipe the codes from her head. Lockdown overheard this and tried to stop the pair, but in the struggle the device overloaded and sent out a powerful burst. In the confusion, Ratchet and Arcee were somehow able to escape... but the EMP blast had wiped Arcee's memory clean.
She was brought to the Elite Guard headquarters, where she was uplinked to a computer system by the Autobot brass to scan her entire memory core for any remnants of the access codes. It was all unsuccessful until Ratchet went up to her, and a beam of data when from her processor to his, giving him the data the Autobot brass needed for their top secret project.
Ratchet later found her in an Autobot infirmary and did his best to prevent Shockwave from getting her. Unfortunately, he succeeded in taking her from Ratchet and took her to Megatron aboard Omega Supreme, where she apparently didn't lose all of her memory.
She then appeared when Shockwave was trying to get the activation codes to Omega Supreme. Her memory was still damaged and randomly went back to her days as a teacher and intel officer.
When Ratchet, Sari, Bumblebee and Bulkhead went to the moon, Sari figured out a way to fully restore Arcee's memories thanks to her newfound skill with technology. She then used the access codes to gain full control of Omega Supreme. After the Final Battle on earth, she returned home to Cybertron with the Autobots, Omega, and the captured Megatron.