Undertale Theories (Major spoilers)

http://thepageofhopes.tumblr.com/post/129688844067/i-think-i-solved-undertale

The short version is that I disagree with the primary theory that is presented, with multiple consciousnesses of the fallen child existing.

Part of this is the fact that Undertale tends to have something, somewhere to explain every element of the game, and this is something that's been projected and has no other theory supporting it in the game data.

The primary reason though, is that everything flows perfectly fine if the Fallen Child only has one consciousness. The only detail is that the Fallen Child is filled with so much hatred that his soul/consciousness is able to persist for extremely long periods of time. It's known that human souls can persist outside of their bodies, but it isn't clear how long it can.

Thus, going chronologically...

The Fallen Child ends up in the underground somehow. He manipulates the Dreemurr family into taking him in as a son, and manipulates Asriel into being BFF's. The Fallen Child then commits suicide, and has Asriel absorb his soul. Asriel knows that the Fallen Child wants to be buried amongst humans, so he breaks through the barrier having absorbed the Fallen Child's soul. Asriel has his soul and the FC's soul, and eventually wins out, not being able to attack humans to get the next six souls necessary. At this point, Asriel dies, and his soul, along with the soul of the FC, separate.

Afterwards, Alphys's experiments cause Flowey to come into existence. Flowey is only Asriel. Flowey starts by being a force of good, fixing everything. There's some residual influence from the FC, but Flowey/Asriel is able to use his power to save/load to exhaust everything. This power, along with being unable to feel positive emotion, eventually is what drives him to the sadistic monster we encounter as Frisk.

Frisk eventually winds up in the Underground, although we're not sure why. What is clear is that the FC attempts to control Frisk. Whether the other six children had to face this battle is unclear. FC's ability to control Frisk is dependent on YOUR actions throughout the game. The FC never had control of Flowey, it only had other humans. Remember, humans cannot take over monster souls, only the other way around. During the Genocide path, Flowey realizing the FC/Frisk's plans are what cause him to have a Heel-Face Turn and warn Asgore. Flowey realizes that the statement "Kill or be killed" doesn't exclude him, which is what causes his change of heart.