Alright, I've been doing some synthesizing, and I'm prepared to offer my full thesis on Fukua as a character. It's all there, Love, Hate, Smooth Criminal, Jello, Two Souls, One Body, Brain Drain, Cloning.
This is my headcanon. Only Word of God can stop me now.
Fukua was accidentally created by Brain Drain whilst he attended to his pet project of attempting to produce human clones—something he was becoming quite skilled at, but still lacked several vital components to accurately produce one. While working on a genetic copy of Filia, complete with parasite, the souls of two fighters, freshly murdered in a brutal brawl not far outside the facility, unexpectedly flew into the body and became its inhabitants, and the clone sprang to life. One, the "once-proud warrior," was a fairly nameless warrior, only notable in the sense that she was a Canopy Kingdom Grand War veteran. The other, the "silent assassin," was an expendable, but suave Medici Mafia hitwoman paid to murder certain people in displays of the Mafia's reach and drive. Her latest hit was to assassinate the old veteran, but unfortunately for her, she still had her military wits about her—the struggle was bloody, and in the end, they both perished.
Initially, the two souls resumed their bloody brawl inside the body of "Fukua" (As one of Lab Zero's scientists had jokingly named her), as they struggled for control over the new body. This, combined with some instabilities the unfinished body was showing, eventually led Brain Drain to dub the clone "utterly useless," and dumped her on the street to fend for herself. He figured she was soon to die anyway; between the lack of cooperation between the two souls and the body's instability, it seemed inevitable.
Brain Drain would later discover that he was wrong about the former half. When they were unceremoniously dumped onto the street with nothing but the clothes on their back, the two souls had to put aside their differences and work together in order to survive. Over time, as they filed their lives into the common mind they now shared, the two learned that they had more in common than they thought. Hatred turned to reluctant camaraderie, camaraderie to lukewarm companionship, companionship to friendship, and friendship to what could only be called "love." Although sometimes, they seem to need some time apart, and one can sometimes get ahead of the other, the two now consider themselves inseparable, both literally and metaphorically. Eventually, Fukua got it in her mind to seek out the girl from whom they were cloned, not realizing that they were about to get swept up in the septannual hunt for the Skullgirl...the prize being a wish.
Alright, now it's time for some footnotes!
> The gelatinous thing atop Fukua's head, nicknamed "Shamone," seems to be a grinning mockery of real organic parasites. Like synthetic parasites (As it technically is one), it is non-sentient, and responds only to the wills of Fukua to attack. Removing it has been discussed between the two, but it seems a better idea to keep it around for now.
> Despite popular belief to the contrary, Fukua has no disdain for Filia, or Filia for Fukua. Fukua is actually thankful to Filia, because, in a way, it's because of her that they were allowed another chance at life, and to meet one another. Were Filia to meet Fukua, she would be happy to meet her clone, though Samson would probably find the whole thing quite fishy.
> The once-proud warrior isn't particularly fond of fighting, but will do it if need be. The silent assassin seems to get some form of enjoyment from it, and sometimes pokes and prods her opponent with taunts. They switch out who does the fighting on the fly, but the Warrior does most of the talking.
> As aforementioned, Fukua's body is unstable, which allows one soul to temporarily separate itself from the body and rejoin later (These actions resulting in a flash and a spark of electricity, respectively). The Warrior isn't very fond of doing this, as she is somewhat paranoid that sometime she may not be able to return, but the Assassin is more than happy to take this risk, especially because it lets her pull some nasty tricks on the opponent. Brain Drain is probably the only one capable of fixing this "defect."