The game seems to hint that - but never outright states - that Akira's older sister is Isabeau. This seems to be further reinforced by the fact that there were versions of the main character that died in both Blasted and Infernal Tokyo but in all three worlds the child kidnappings happened. The main character apparently looks similar to his former self and it seems likely that Isabeau would look similar to Akira's sister. In fact, the oddity of this is that Isabeau, Walter, and Johnathon might be the children who were taken  and simply brought-up to believe that they were part of the Eastern Kingdom of Mikado. That might be stretching it; but the hints are there. Third Akira likely became Aquila but there seems to be strong evidence indicating that Isabeau is or was his mysterious sister. 

Also, considering the ICBM events, Yuriko, and Masakado. It seems highly likely that SMT4 Hero is the reincarnation of the SMT1 Hero. An alternate version of the SMT1 Hero but still a reincarnation. I know, that sounds horrifically confusing, but THAT evidence is certainly concrete enough to point out. The Truthful Knight (talk) 02:02, March 6, 2014 (UTC)

First off, Shin Megami Tensei IV is not in continuity with Shin Megami Tensei. At all. Not in the slightest. The ICBM's were launched for a different reason and the whole thing had different outcome (hell, three different outcomes), Masakado...who was basically a nonentity in SMTI's plot who existed solely to give Futsuo a powerful weapon, while he is crucial to the plot and backstory of SMTIV, and Yuriko's motivation is different from the SMTI Yuriko's. (Hell, it's implied that the SMTIV Lilith is flat out a different Lilith from the Biblical one we see in other games.) Shin Megami Tensei IV is set up to initially bring to mind the first two SMT games (the four main characters are set up and named to bring the mind the four main characters of SMTI, several of the villains are set to remind you of SMTI villains, Mikado brings to mind Tokyo Millenium, etc), but as you actually get into the game, you realize that SMTIV has it's own backstory and plot independent from the earlier games, and attempting to reconcile them is nigh impossible.
As for Isabeau, nothing of the sort is implied. Yes, it's stated that Akira had a sister who was abducted by the angels. What comes of this? Well, in the "main" timeline, Akira goes topside looking for her and later betrays the other "Tokyo" humans and sides with the angels, becoming Aquila. That's it. The fate of the sister beyond her abduction and it's effect on Akira is never mentioned outside of Infernal Tokyo, where it is mentioned that she starved to death. If Isabeau is related to this sister in any way, it's never brought up (and indeed, I don't think Isabeau is implied to be "special" like the other three at all).--Otherarrow (talk) 16:06, March 6, 2014 (UTC)
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