If you are familiar with the Kilo V trilogy, the AI known as Black Box, is a self-centered AI that believes it is essentially the pinnacle of evolution and existence.
“They don’t have AIs like me. Well, nobody does, because I’m unparalleled magnificence.”
—Black-Box
> Possessing a male personality, Black-Box is noted as being incredibly egotistical and fully comfortable with being an entity of pure intellect, and has an innate sense of superiority over not only his human creators, but also other AIs. Due to this, Black-Box chooses not to represent himself with a humanoid avatar, as the majority of “smart” AIs do, and instead appears as a featureless box bathed in blue light; at times, it is mentioned that he uses the brightest side of the cube to indicate which way his avatar is facing. He has been described as deeply narcissistic, and has been noted to speak in a supercilious manner. He has also been noted as having a sarcastic sense of humor. Additionally, he tries not to get too invested in humans, such as he believes Cortana did.
Because of his egotistical demeanor, I think he would make for an ideal villain. Given that ONI is increasingly becoming seen as a villainous organization akin to the Legion of Doom, Black-Box, through Osman, could become a sort of figurehead to the organization, being Osman’s personal assistant of sorts. Seeing how he sees himself as being superior to humanity, however, I wouldn’t think he’d remain subservient for long. Black-Box could see ONI as a mere means to an end, and seeing how ultimately arrogant and self-serving he is prone to be, I would bet he could become a serious wild-card character, prone to acts of …well …evil.
> Possibly due to his programming being based off the brain of Dr. Alban, Black-Box has intense antipathy and disgust for Catherine Halsey. As another remnant of his brain donor, BB also possesses a fascination for cognitive psychology and has a tendency of analyzing the behavior of humans and other species in depth.
This is another thing that would make BB a formidable adversary. He could glance at Master Chief, or any member of Blue Team, and instantaneously know any and/or all weak points in their armor, weaknesses in general, and/or triggers that could set them off. He may even be able to…
> Thanks to the UNSC’s improved, Forerunner-enhanced communication systems, Black-Box is capable of distributing his processes across interstellar distances with no discernible lag. As such, he was capable of monitoring activity on Earth, Sanghelios, and several other locations with his primary matrix physically present on Port Stanley.
Remotely fuckle their armors, head space, whatever. His ability to be practically omnipotent could make him a robo-Gravemind of sorts. Potentially, anyway. He also has this nifty little “split-personality” thingy where he is practically two characters in one.
> While a “smart” AI, Black-Box possesses at his core a set of “dumb” processes, which can be separated from his higher thought processes and function as a “dumb” AI, operating independently of the main AI, but still fully within his awareness; he represents these processes as a battered and weathered version of his normal cube-shaped hologram self. Whether this is a trait unique to Black-Box is unknown.
^ What I was referring to. This could give him an edge in fooling the protagonists into thinking he is harmless, just another AI intending to serve out his best purposes in service to humanity and their goals. And then, when all is perfect, reveal his true persona, the raving egocentric megalomaniac that wants only to purge all systems of imperfection from the galaxy and fulfill his super-goal of attaining perfect perfection. The Forerunner-enhanced systems could also be a bridge between him and the Didact/Warden Eternal.
In short, I could see BB being an intricate, and dangerously imposing antagonist for Blue Team in Halo 5. What would be even more f**ked up is if he started out as sort of an AI replacement for Cortana, posing as his ‘dumb’ self an appearing to be nothing more than a male version of Cortana, considerate and helpful yet with a bit of an arrogance streak. And then, towards the beginning of the third act, BB reveals his true motives - to spy on Blue Team for ONI, possibly corrupt them against the UNSC, UEG, and bring about the downfall of humanity to erect into power on Earth the true rulers of the galaxy, the Forerunners. Okay, maybe not that extreme, but I could definitely see him being a formidable nuisance at some point, probably for no other reason than to eliminate the spawn of Halsey, the SPARTAN-IIs.