As copy/pasted from
rp_anon_meme in response to concerns.
Since I know this pairing has been unpopular with anon this time around -- and I can completely understand that, and I'm sorry so many are unhappy with it -- I can attempt to offer an incredibly tl;dr explanation of it, even though I'm sure it won't change everyone's (or anyone's) opinions.
Essentially, Charles and Soubi are both really needy characters. Soubi needs to feel useful and wanted, and Charles needs to feel like he belongs somewhere and can be accepted. Both of them are also strongly defined in their actions by the characters around them, and both of them previously shared a physical and emotional bond with a master who had thrown them away -- Soubi as Seimei's Fighter, and Charles as Diva's Chevalier.
When they first arrived here, Soubi was alone without Ritsuka or Seimei or even Kio, so he was pretty much back to being a "living corpse", as Kio described him once in canon. Likewise, Charles literally was -- he'd just been brought back from being dead, something he'd wished for for decades, after being betrayed by the person he'd depended on the most and after watching the only person who had treated him kindly die because of him. He was an empty, broken mess when he got here with none of the fire he used to have.
Soubi made a post about his dislike for butterflies at one point, which caught Charles's attention because of his similar feelings toward moths -- both of these have strong symbolism for these characters and their relationships in canon. They developed a sort of mutual understanding from this; they also spoke about scars and eventually showed theirs to each other -- we'd planned to log all of this, but computer trouble at the time unfortunately prevented us from going into as much detail as I would have liked.
They found they liked spending time together, but it wasn't as simple as them genuinely liking each other; despite the basic understanding they had, they were both using each other as replacements for what they were missing. Soubi, like Van, was tall, wore glasses, gave Charles compliments and obeyed his requests; Charles, like Ritsuka, was small and young-looking with similar hair and eyes. It turned into a sexual relationship entirely too quickly because they were both trying to fill the hollows of these aimless existences with something else, and it became serious afterward not because they really loved each other but because they became so dependent on each other for making them feel like they were needed and wanted that they deluded themselves into thinking it was something more.
On Charles's part, at least, I do think this was in character. In canon, he was not only emotionally unstable and highly conflicted; he was incredibly physically and emotionally dependent on Van -- he literally tore out his heart and offered it to him when he thought Van was leaving him, an action that carries too much symbolism for it to be simply an offer of blood for experimentation. In the last volume, Van laughs about how much Charles used to cry to him about how lonely he was, like an annoying animal, and how easy it made him to manipulate. Charles also became attached to Riku incredibly quickly, and was strongly affected by Riku's kind and accepting attitude toward him, something that Solomon commented on. So, given his canon history of quick attachments, intense emotional dependence and loneliness, and a talent for deluding himself, I think this was a reasonable path for him to take, particularly given his emotional state at the time.
As time has progressed, Charles has come to terms with the fact that he doesn't have a choice in existing again and has gradually attempted to become more like his old self, although since everything that happened with Riku, he's finding that most of his former motivations are conflicted now, and everything he says is still largely hollow. Soubi has played a significant part in this, though -- Charles has someone who caters to his demands again, someone who is loyal to him and likes him, and this small bit of power has enabled him to at least attempt to recover his former facade of cold superiority. Unfortunately, this also means that his dependency, possessiveness and paranoia have taken a turn for the worse, so in his efforts to show less weakness, these more often manifest themselves in abusive behavior now. On Soubi's part, on top of the lonely youthfulness of Ritsuka, Charles has increasingly come to show more of the possessive domineering qualities reminiscent of Seimei, which complicates the feelings on both sides.
Basically, this has always been an unhealthy relationship, and has never been intended to be read as anything else. The characters are terribly dependent on each other (on top of the endless emotional complications, Soubi has been Charles's primary food source this whole time, so it extends to physical dependence as well); when they say they love each other, they're being honest, but they both have completely skewed and unhealthy conceptions of what love is -- they hardly even speak to each other on any sort of meaningful level, and they don't actually know each other well at all. They forced something unnatural from the beginning because they needed it. Soubi is still more devoted to the idea of Ritsuka than he is to Charles in reality, which I'm sure will become more apparent now that the game has a Ritsuka again, and I doubt it will end well.
So I'm sorry if we're displeasing other players, but there honestly has been a lot of thought behind the constantly changing emotional motivations in this relationship, and it has evolved significantly from where it started seven months ago. It is intensely dramatic and emotional, but I think that's just inherent given the characters involved.