Breq is the main character of the Imperial Radch trilogy. She is the last remaining part of Justice of Toren, specifically its ancillary One Esk Nineteen, but upon the destruction of Justice of Toren she became confined to a single body.
At the time of Ancillary Justice's opening, Breq has been living as One Esk Nineteen for nineteen years, three months, and one week. She has never entirely gotten used to having her consciousness restricted to one body, and frequently reaches for parts of her awareness that are no longer there.
Breq frequently describes herself as having inhumanly fast reflexes and an eerily expressionless face and voice, when she is not overtly trying to mimic human expression. It is suspected, however, that she is not an entirely reliable narrator on this count. In several emotionally intense situations, she will not describe herself as reacting emotionally or having any particular emotion, but tell-tale pauses in conversation or descriptions of other characters wiping away the tears on her face will belie her true feelings.
As Justice of Toren, Breq had a strong love for Lieutenant Awn, and decades after Awn's death will stop at nothing to avenge her.
She has a great interest in songs across the millennia and universe, and will "collect" songs from cultures that the Imperial Radch comes into contact with (and frequently annexes). She is more or less constantly humming and singing them, to the consternation of everyone around: although they often decide they prefer it to her silence, since silence indicates that something is not as it should be.
It is implied that the body that Breq inhabits now is the one that was attached to her ship's consciousness in Ancillary Justice after action on Shis'Urna. Justice of Toren mentions that the body chosen by Medic didn't have a singer's voice, as the bodies she prefers do, and this fact (Breq's toneless and bad singing) is frequently referenced throughout the trilogy.