Woman I'd take into a fight: Jaina Solo (Star Wars Expanded Universe)
1) She's a Skywalker with a prophecy. Her grandfather was the Chosen One; Jaina is the Sword of the Jedi:
Luke Skywalker: I name you the Sword of the Jedi. You are like tempered steel, purposeful and razor-keen. Always you shall be in the front rank, a burning brand to your enemies, a brilliant fire to your friends. Yours is a restless life, and never shall you know peace, though you shall be blessed for the peace that you bring to others. Take comfort in the fact that, though you stand tall and alone, others take shelter in the shadow that you cast.
Some days this is easier than others. On the bad days, she has to do things like face down Sith lords or choose duty over love, and she grits her teeth and does what is necessary.
2) She's the daughter of Princess Leia and Han Solo, which means she's genetically badass on both sides and that her embroilment in the struggles of The Galaxy Far, Far Away began even before she was born, which made for an interesting childhood. Like her grandfather, Anakin Skywalker, and uncle, Luke Skywalker, Jaina showed a great aptitude for piloting and mechanical things from a very young age.
3) She began flying in combat when she was only sixteen. She even led Rogue Squadron for a time.
4) She's pretty damn handy with a lightsaber, too, despite being a little bit of a thing like Leia and Padmé.
5) She knows that if the terrifying, trillions-murdering hordes of Force-resistant galactic invaders ask if you're a goddess,YOU SAY YES. Then you make them believe it.
6) When Jacen Solo, her twin brother and only surviving sibling, fell to the dark side and starting calling himself Darth Caedus and choking people on the bridge of his flagship and corrupting their cousin Ben Skywalker and burning planets and killing their Aunt Mara, and much, much more, and Luke Skywalker told Jaina she was the only Jedi strong enough to kill Darth Caedus who could do so without falling to the dark side in the process, what did she do?
She trained hard and then took care of business, that's what she did, with willingness to give her own life if necessary and despite the great emotional cost.
For all these reasons and many more, this is the woman I'd want at my side in battle: Jaina kriffing Solo!
(Cross-posted to awesome_women, which you should join!)