Why are they so similar?
Inubushi Keiko and Yukidoh Sayaka share a few odd similarities. First of all, they both have DID, and have a personality with murderous urges. The two of them share the same blood type (AB), and both have black hair. Their birthdays are different, but if we assume that Sayaka had survived, she would be about the same age as Keiko. If Keiko’s birthday is different than what’s listed in her profile, then this inconsistency would be cleared up.
Basically, what I’m trying to say is that Inubushi Keiko and Yukidoh Sayaka are physically the same person.
The truth of Yukidoh Sayaka’s death
There is one problem with the theory that Inubushi Keiko and Yukidoh Sayaka are the same person; namely, the fact that Keiko is alive while Sayaka is dead. However, this is easy to explain:
Sayaka’s “death” was the death of the personality named Yukidoh Sayaka.
This explains their differing birthdays, as well as why they have different names. If the government looked into it it’s possible that the truth would come to light, but Leiblich Pharmaceutical’s political influence could prevent that. If you think about it that way, it’s likely that Leiblich itself caused Sayaka’s death. In fact, that may be the reason why the real Yukidoh Satoru got a job at Leiblich Pharmaceutical.
Inubushi Keiko and Mayuzumi Lin
In the epilogue of Satoru’s route, Satoru sees Inubushi and Mayuzumi talking and thinks it’s strange. It would be strange, since the two of them should be complete strangers to each other. Plus, Mayuzumi is not the type of person to chat lightheartedly with some random person. But if you assume that Inubushi Keiko is Yukidoh Sayaka, this problem disappears.
Mayuzumi’s ex-boyfriend was Yukidoh Satoru (sunglasses), and Yukidoh Sayaka was his sister. Sayaka and Keiko occupy the same body. To Mayuzumi, Inubushi is her ex-boyfriend’s little sister. If you think about it that way, it’s not strange at all to think that they might know each other.