Transmigration: Rey’s body was designed after Fiona Cleary, Leland Cleary’s daughter who had died during a typhus outbreak in Ireland. She is a spitting image of that girl, had she lived into adulthood. Her flesh and blood and fibers are cloned from a “father” and “mother” creator, Lucas Coffey and Undine Stransky, making them biologically related by all counts.
She is, however, designed specifically for the sake of combative purposes. It was the only way that Lucas and Undine could have had the funding to produce her and the other Pieces (synthetic humans) at all.
Not only does her organic cybernetic physical composition make her more durable than a normal person, as a Piece it is also possible to reassemble her, or transfer her “soul” to a different body after the destruction of her current one. When reborn, her memories are reset, but her experiences are imprinted in the core of her coding. This means that her sense of self is deeply buried, but not entirely rewritten, only stored.
Update: Rey has made a point with her father-creator, Lucas Coffey, that he is to not create any more vessels for her to transfer to. As such, she is to remain in her current vessel until it dies.
Brísingamen: The Brísingamen is an energy stored within Rey’s vessel that, once imprinted into her chest, supplies her bionic physiology with the ability to function. Her skeletal structure is made up of fervidium alloy, a type of glowing metal that constantly burns from within once activated by the Brísingamen.
The Brísingamen can unleash heat particles from the vessel, generating enough to cause a person to burst to flame. When activated it takes on the form of neon red veins pulsating through the parts of the body Rey is applying the energy to, which can occasionally cause her skin to secrete a dripping, oily, extremely hot substance.
Basically, she’s a walking powerhouse. This also grants her an immunity to fire and extreme hot and cold temperatures, though it doesn’t necessarily protect her completely because her hair can burn off.
On the Tranquility, the Brísingamen is only capable of radiating no more than a few bursts of flames before wearing her vessel down.
Update: At this point, Rey has a better understanding and control of the Brísingamen. She even once made use of it to help cauterize an injury after someone had lost his arm, being able to manipulate her own body heat.
There is a massive downside to it, though: If she overexerts herself, takes on too much (say, a whole battalion of people) with her Brísingamen, then Rey will burst into flames. Even though her body is immune to fire, she will then continue to burn until extinguished by an outside source. While carbon dioxide, extreme cold, or a large quantity of water should do the trick, it’s best to use a method that requires distance between Rey and anyone else, because by then she is literally on fire.
When using the Brísingamen, red glowing veins pump through wherever she is directing the energy (i.e. her arms). Utilizing it at its fullest capacity (say, the rate it would need to burn a person alive) results in her skin heavily secreting a black, oily substance that has been dubbed samandrine. Think of this substance as the oil that keeps a lantern lit. Samandrine can be extremely toxic when ingested, resulting in temporary paralysis and inability to breathe. It’s not always fatal, though like with any poison it can if belted out in high quantities. Just don’t drink it and you should be fine!
It is possible for her to be able to focus on the Brísingamen in her arms, and “retract” the flames to have better control over it. However, she hasn’t had much time to practice this properly while her mental state was still at risk.
Here They Come to Snuff the Rooster: Military training exceeds well beyond that of the average soldier. Her skills range from long-ranged firearms to melee weapons and close-quarters combat. Her specialty lies with assault and sniper rifles, handguns, bayonets, and combat knives, as well as disarming her opponents.
Like many in the military, she also has some knowledge of basic first aid.
It should also be noted that Rey now carries the knowledge of nearly a century’s worth of combat experience. Her lives spent as highly skilled snipers, soldiers, and special operatives carry over to her present vessel.
Built Ford Tough: With an almost superhuman endurance and equilibrium, Rey’s keen sense of movement allows her to be heavily skilled in being able to run through environments via rolling, dodging, vaulting, and climbing. She also a higher strength and pain threshold than the above average human, which can be obvious from her well-toned physique.
As mentioned above, Rey also possesses a bionic physiology. She is made up of fervidium alloy, a type of glowing metal that can be activated at various degrees of heat by Rey’s Brísingamen.
The alloy makes her more of a rock than anything. I would place her at around 140 kilograms, due to the fervidium metal containing her Brísingamen.
Moreover, her skin and organs are cloned from the DNA of both her “mother” and “father”, Dr. Undine Stransky and Lucas Coffey. Due to the immortal strain of cells belonging to her father, also known as LUC-156 or Cleary cells, Rey has a natural ability to heal faster than the average human. But due to the mixture of the human and immortal cells, she is not quite the same as her father, and as such her healing has some limitations. For instance, broken/fractured bones will mend within a matter of days, but she is unable to reattach lost limbs or repair scar tissue.
Bablefish: Due to her now-merged identities, Rey had a bunch of languages downloaded into her memory, making her multilingual. She is fluent in English, Spanish, Zulu, Dutch, German, Russian, Italian, Arabic, and Hebrew. Thanks to the ship's nanites, however, this probably won't come across that clear unless she’s using some really bizarre curse words.
She’s Not Heavy, She’s My Girlfriend: In addition to her mass, the fact that Rey is an extremely heavy piece of equipment doesn’t make her very stealthy. She has no problems with movement and speed, but once she’s running, it does make it difficult for her to be very stealthy.
Being Human: More often than not, Rey does not know how to handle her emotions, and tends to either react impulsively.
Her destructive personality disorder, as seen in her personality section (and here), can be seen as a weakness in itself. While compliant to commands at first, the more independent she becomes, the harder she is to control.
Update: In order to eliminate the threat of the DPD, Rey was restored with all of her previous memories. Overwhelmed by it all, she has learned about grief, and sent her down the path of learning as well. At this point, she has memories and emotions from the experiences of her past lives before becoming self-aware of her role as a creation of Project Seraphim.
Fragments: Because of all the memories that had been crammed into her head, Rey has... identity issues. A lot of them. After a while when she starts to remember some things, she has difficulty differentiating between which one is her own, and which ones were fabricated and programmed into her head. It’s all one and the same to her, either way. Who cares if she was the daughter of whores and serial killers, right?
Because of this, she might get various details of her backstory mixed up, be they real or not. The details may be so subtle that it should take a perceptive individual to take notice it.
Be it a flaw in her coding or posttraumatic stress, Rey’s mind is shattered, oftentimes suffering hallucinations and flashbacks. Sometimes, the mental strain can be so harrowing that she may mentally “shut down”.
Update: At Rey’s current point, she is now technically developed a sense of photographic memory. Nearly an entire century’s worth of memories in addition to fake ones are now restored. What more, Rey has difficulty with her perception of time. One minute she may be having a perfectly normal conversation, but a simple thought can have her mind spiraling backwards into a war zone or a distant memory. Most of the time she has no way of telling how long she has been trapped in a memory, or what timeline she is actually in. Lucas Coffey had educated her in ways to keep her mind focused via mediation, though it’s not a skill that she can always maintain; given that the mediation is a product of his Jainist lifestyle that she can’t even begin to comprehend.