Oh my god, what if Phi is the daughter of Sigma and Diana? O:
I’m assuming you’re saying this in regards to this post! While that is still a plausible theory, and a surprisingly common one at that, you also have to consider that a mother could and would leave their daughter a locket with their birthright pictured within it. It could be that Phi is Diana, and that her mother had left her a locket picturing the very woman she was named after. Both theories are still in the air and incredibly plausible at this point in time.
Although, it is important to remember that no family member in Zero Escape has been mentioned without having some sort of relevance so...we’ll have to see!