ANGEL episode Waiting In The Wings then you will see that it is none other than Summer Glau. This isn't really a surprise since trained as a ballerina and therefore looks completely at home in the part.
From this small background role, however, started a career that has made her such a genre icon that she was able to appear as herself in a cameo role on the best geek comedy show ever THE BIG BANG THEORY.
Glau manages to combine a vulnerability and apparent fragility with inner strength and this is the combination that made her perfect to play the kick ass robot sent back in time to save John Connor (again) in TERMINATOR: THE SARAH CONNOR CHRONICLES. There is a definite kick to be had watching this slight girl beating the living daylights out of some big hefty blokes.
It was, though, just a variation on the part she had already played in Joss Whedon's sadly truncated FIREFLY in which she was a young and fragile girl who could turn into a kick ass killing machine when the plot required. In an ensemble as strong as this show boasted, being able to hold her own proved her acting ability.
Since then, she's gone on to play one of the empowered 4400, a supergeek scientist type in DOLLHOUSE and the mentally fragile superhero sidekick in THE CAPE (just coming to an end on SyFy).
No matter what she appears in, Summer is watchable and eye catching and a perfect science fiction icon.