Cru is a Brood-hunting alien created by her people to fight and destroy the Brood. Cru's race were (as far as we know) a peaceful people that lived on a planet far from Earth. After a Brood attack that almost completely wiped out their population, Cru volunteered for an experiment that would turn her into a weapon capable of fighting the Brood. Scientists from Cru's planet used their technology to try to make her into a creature of war, and they succeeded. As soon as the process was finished, she attacked the scientists and eventually went on her task of hunting down and killing the Brood.
Cru was originally an enemy of Carol Danvers (Ms. Marvel) but after a time it eventually became more of a symbiotic ally. In its first appearance it was attempting to acquire the Cavorite Crystal and use it as a power source. The Brood, in an attempt to save themselves, convinced Cru that the Cavorite Crystal would be a good power source since Cru channels radiant energy. (The Cavorite Crystal can create things like faster than light star drives and anti-gravity devices with just a little energy, but it's unstable so too much energy makes it dangerous.) The Brood hoped when Cru tried to use it, it would destroy both Cru and the Earth. The plan failed and Cru destroyed the Brood and when it got the Cavorite Crystal, it didn't die. Cru effortlessly destroyed a military base designed to withstand nuclear strike, got the crystal, destroyed the surrounding town and flew into space where it was confronted by Ms. Marvel. Cru had integrated the crystal into its body so Ms. Marvel reached inside of its gun arm and blasted energy at the crystal, creating an explosion that blew her back to Earth and seemingly destroyed Cru. After the explosion, pieces of Cru started to reform in space.
Some time later, after Ms. Marvel had become leader of her own SHIELD task force, it was revealed that Cru hadn't died and was actually inside Ms. Marvel. During or after the explosion Cru had somehow gotten inside Carol's body and was making her physically stronger, more durable and gave her a healing factor that was, according to Beast, beyond even Wolverine's. Soon after Beast ran his second battery of tests on Ms. Marvel, Cru reappeared. A fight would re-ensue until the assailant would probe Carol's mind, revealing her identity to Ms. Marvel as well as her past and how the entity known as Cru came to be. Also explaining that when Cru last dissipated she found pieces of her augmented physiology were missing, somehow having spliced into Carol's genetic structure when the Cavorite exploded; explaining how the latter retained a healing factor superior to wolverines. While admitting mistakes had been made and that she could have done things better, Cru took both herself and Ms. Marvel to the primordial landmass called Monster Island.
All of Cru's people are alien cyborgs, but through experimentation Cru has been made into a living weapon. She is a shape shifter that can grow and regrow limbs, blades, heads, tentacles, guns and even regrow its entire body from its own dispersed goo. According to Cru, technology and biology are the same to her people, which explains how she can literally grow machines out of her body. Its machinary, like the rest of its body, is controlled by its mind and it can create more parts if necessary. Cru can absorb energy as well as blast it through built-in lasers. It is a powerful psychic and communicates telepathically, able to painfully rip images/memories/thoughts out of the minds of others.
While bonded to Ms. Marvel, Cru showed the ability to break Carol out of mind control from various enemies and was able to almost instantly heal her from attacks that would normally have killed her. Cru was also able to remain hidden (except when she was healing Ms. Marvel) from even the most advanced technology Beast has.