Seven was one of the ultra-rare alpha females. She was small but quick and was the death of Duke Williams' best friend. She died by the hands of Aidas (an augmented albino sangheili) shortly after.
1) Though it is said that alpha males have four arms, alpha females do not. Instead, their system is enclosed, making it harder to attack their weak points or sever their brainstem. Due to their system being enclosed, they are unable to fire their "mouth lasers". Instead, all that harnessed energy is transferred to their speed and agility, making Seven the fastest known creature alive in history until her death.
2) Seven is a mute, which adds to her "assassin" demeanor.
3) Seven will kill any male that she senses, but won't dare lay a finger on any female. No one knows or knew why.