Credit to @themaverickhunters, @rebelreploid-dynamo and kashtara for helping me work through a stream-of-conscious thought process to work this out.
Issimo was designed at his creation to house multiple CPU chips, originally for storing the massive amounts of media data owned and produced by Resonant Records, as his official capacity (besides being Octavia's son and heir) was a walking data repository. These CPU chips hadn't yet been needed until Issimo was brought into the 9th Special Forces unit, and thrown into the world of covert ops.
In this new line of work, the extra, unused CPU chips were used to house basic AI data instead, for use as aliases for long-term undercover work. Two of the more personal aliases - Viking, the persona Issimo adopted at Black Lake in honor of his brother, and Acoustikat, Issimo's pop-star alter ego - slowly evolved from basic AIs into fully realized sentience over the course of Issimo's near fifty year career with the Maverick Hunters.
As no other alias has yet to display this level of growth (or any growth, really), this is probably due to how much time and attention Viking and Acoustikat specifically were given outside of Issimo's work-related missions, which all other aliases were confined to.
Dark is a notable exception, as he was originally a fully realized AI whose IC chip was removed, and planted inside Issimo's own head to be used like a puppet.
