So I've been wondering for a while what's going on, exactly, with Flowey, because something about him just doesn't add up at all. We know that robots are made of metal and magic, but what is Flowey made of? The part that I feel most opposed to is the claim that Flowey does not have a soul. This is a fairly typical stance because Flowey is kind of a jerk, and is commonly said to be emotionless, and because Alphys said so. The first part is irrelevant, the second is incorrect (Flowey does feel emotions, just not positive ones; he frequently expresses fear, frustration, and lots of anger), and the last part… well, if you've gotten this far and trust everything Alphys says… you're doin' it wrong. Besides, how COULD she know? She didn't even know that the flower she chose was the one dusted by Asriel's ashes. She thought it was just another flower and picked it at random. She just assumed that it didn't have a soul because flowers typically don't.
Either way, it doesn't make sense that a mere flower could suddenly gain sentience simply from Determination. Determination is a power source, not a mind or soul. It wouldn't make it move. As someone else pointed out in another thread, how could a soulless being that's not a monster even be able to use magic attacks in the first place? Monsters are made mostly of magic so their souls are better attuned to it. Their souls essentially are their bodies, and thus are their magic. This is why their bodies and attacks weaken when their souls lose the will to fight. If Flowey had no soul, and no monster body, where could his attacks come from? There wouldn't be any magic to use.
But the most important reason why I can't accept the soulless-Flowey hypothesis is because it would mean the child you encounter at the end of the game WOULD NOT BE ASRIEL! It would be just something like a copy, clone, or echo of him given some weird agency and somehow pulling his memories from the aether.
It's said that the "Boss Monsters" have souls that can persist for a moment after death, something that doesn't happen with other monsters. Considering that they're the defacto rulers of the Underground, this might just have something to do with a kind of innate strength or power. The more powerful the monster, the longer the soul can last. Strength of the body is strength of the soul because they are close to the same thing. By absorbing Chara's soul, Asriel gained exponentially greater power, and so perhaps his soul could persist for exponentially longer after death. Long enough to sit inert on that flower until it was injected with Determination, maybe. Then, it wasn't until he absorbed the human souls and all the other monsters' that Flowey was able to recreate his physical body and regain a vessel that could process positive emotions, and thus returned to being completely Asriel, body and soul.
Well, at least that's my theory on the whole thing. It makes me feel better knowing that Asriel is still in Flowey somewhere, that Flowey isn't just some corrupted copy that carries his face around like a death's mask. After all, at the end of the Neutral Ending, he DID tell me how I could go back and change things so as to fix what happened. Was that just Flowey manipulating me into putting the pieces in place so he could gain the equivalent to seven human souls, because clearly six wasn't enough to defeat me? That's entirely possible. But I choose to interpret that as kindness, instead. A little bit of the old Asriel peeking out from behind Flowey to help us both get our "happy ending". Kindness doesn't come from dust; it comes from the soul.
