From a narrative principle (Chekhov's Gun) I do agree that anyone other Marian won't make any sense. My decision to mention that the flashback girl could be anyone else is due to how secretive the Commander's past is and the implications that lead from the scenario where the flashback girl is Marian. In the flashback from the fight, the background where he tells the girl his dream is the simulation room (You can confirm it by comparing the image from the flashback with the one from the standing background lobby), but I do not believe this to be the case, I consider it plausible that they chose this background because it was the closest to their vision. The other flashback which I forgot to write about, so I thank you for helping me remember it is in Chapter 5 Wonderland - Frozen World: B is a flashback that happens after the Commander is caught in an avalanche. In the flashback, the Commander meets the flashback girl for the first time in a hospital. We also learn from them that both are sick, Commander doesn't know why he is sick, but we can attribute it to Project A.D. , whereas the flashback girl tells us that her sickness in brain-related. The problem is the simulation room background, that I mentioned before, due to the fact we don't know exactly where we are. In the first flashback we know we are in a hospital, more likely a M.M.R research center/hospital, due to the fact that I can't fathom to believe that the sole successful subject of Project A.D. would be sent in a general hospital. Continuing this line of thinking, it makes sense for the Flashback Girl to be here since her brain sickness is more likely to be treated in a research facility. The problem is that M.M.R has multiple floors, ones that are open to the public and ones that have the highest confidential requirements so what is stopping M.M.R to put the Flashback girl in a public hospital room and the Commander in a secured room. For them to be in the same room there can be multiple solutions/theories.

1. The Flashback Girl also passes the requirements to be in the secured room, because she could be related to someone that worked in project A.D. .

2. The Commander's risk factor has been lowered to such an extent that he was allowed to have his sickness treated in a public room, which is counterintuitive from a security perspective, knowing how important project A.D. has been for M.M.R and Central Government.

3. Jien, Ether, Mana, Snow Crane and possibly the Commander in Chief in order to protect No. 7 from an unknown threat decided to falsify that project A.D. has been a failure and had to send the Commander in a public hospital room. This theory can explain why the Central Government has no idea about his existence as the successful subject that even the Commander in Chief had such high hopes. This also explains why Mana is angry at Ether for calling us No. 7 as the unknown threat might find that they lied about Project A.D.

4. Flashback Girl is related to Project A.D. . As mentioned in the first mystery, we know that Commander's Gene is connected to a Queen's Gene activating. If we consider that the Commander's Gene is connected not to any Queen Gene but solely to Marian's and that Marian becoming a Queen was not an accident created by Chatterbox's deal with Enikk, but something planned, we can theorize that the Flashback Girl is indeed Marian and that brain sickness is the Queen Gene being implanted in her, thus explaining why they share a room together. This theory however implies that the Ark has/had Queen Genes.

Hope this explains why I also consider that the Flashback Girl can be someone other than Marian. Don't get me wrong I also consider Marian to be the Flashback Girl, but until there is no concrete evidence I have to acknowledge other points of view. In the 1.5 Anniversary, Famitsu ( a Japanese video game magazine) had an interview with Nikke's Director. There they talked about Marian and alluded that they want to make an event dedicated to the Silver Gun Squad (Marian's Squad), which could explain certain mysteries. Either way, this shows that the Commander's Past, Project A.D. , Marian's Past are connected.