The Engineer is the resident
technoweenie of the superhero
cabal known as
The Authority. She's also one of the few that prefers to go by her real name, which is Angie Spica. She is no longer entirely human; her predecessor (also called The Engineer) was a
super-villain who fought
Stormwatch and lost. He had been a scientist who worked out some very revolutionary things regarding
nanotechnology; before his death, he
uploaded all his research into the computer of one of his students - Angie Spica.
Angie took his work and ran with it. As she likes to put it, she "gave up her blood and replaced it with several pints of microscopic machines." These machines (the nano) act as her blood, transporting oxygen around her body. However, this is the least of their abilities. Under her conscious direction (not necessarily control; there's too many of them to directly control) they can exit her circulatory system and modify themselves and her into all manner of things. Typically, she is seen encased in a brownish metallic 'suit' of the nano; in it, she is as near to invulnerable as makes no difference. She can survive vacuum, fire, impact, all that good stuff.
It doesn't stop there, however. As far as we can tell, the nano also gives here the ability to interface with near any kind of machine - The Carrier, for instance. Angie is The Engineer aboard, mostly because she is the only one who knows what The Carrier is saying or how to fix and really run it - although others can control it; we see the Midnighter use it at the end of the first arc, to lethal effect.
Angie isn't entirely sure of the limits of her nano - at one point, she joins Apollo in Lunar orbit by having her nano transform her into a rocket-propelled craft; her childlike delight at this is one of the most appealing things about her character. As her new 'systems' count down and launch her into space, Angie (one of the only members of The Authority who always wanted to be a superhero) carries out old fantasies by reciting bits of the Apollo Program checklists, to the amusement of her colleagues - especially Apollo, who is waiting for her.
Finally, she can use herself and/or the nano as weapons. Her usual modus operandi in combat is to extrude large automatic projectile weapons from her forearms, much like The Mask producing the ludicrous armful of weaponry in that film. At one point, beset by several hundred airborne opponents, she divests herself of the nano entirely for several seconds - and has it form itself into an invisibly fine mesh, which neatly Cuisinarts the onrushing clone army.
In both the main and alternate timelines, Angie eventually becomes romantically involved with Jack Hawksmoor, although it's quite likely she dallied with other members of the team - notably the omnisexual Jenny Sparks.
As others have said, if you're at all a fan of the superhero comic genre, do yourself a favor and read everything Warren Ellis has ever done - especially Planetary, Stormwatch and The Authority.