I think they are just really afraid of messing with Kaf's iconography and status as a digital figure and artist, and tbh its understandable. Kaf has a super distinct sound to her voice, and she is already a "virtual idol" in her own right. Design wise, KAFU just looks like a recolor of Kaf's already existing model. KAFU in her deeper voice would probably confuse newcomers to Kaf's music. Its so messy, it makes me question what their original intention and vision for KAFU truly was.
If they want KAFU to succeed on her own by making her the new big "Vocaloid" on the block, they need to really make her as appealing as possible, but if she is to be just an extension of Kaf, she really needs something else to distinguish herself from Kaf, because as it is, trying to explain the situation to people who don't know very much about voice synths sounds like it'd be confusing.
Its really also interesting, because in the scope of major voice synth releases, this is the first time ever where one wholly digital figure voices another, and we can now see the issues that would arise in such a case. Celebrity singers in the past like Gackt and Lia never had to worry about these issues because Gakupo and IA were so distinct in both sound and character, and they both had very different fans and demographics that they appealed to. KAFU and Kaf are both on the exact same media platforms, both sound and look extremely similar, and they're both gunning for the same target audience. What can even be done at this point? Can they both achieve and maintain success without hurting each other? idk