Apparently five years wasn't enough for her to forget about the one she couldn't save... Stupid human.
"What are you doing back down here?" the flower sneered, not even bothering to turn and look.
"I'm taking you home."
"Home? Hmph. I AM home. I told you, I'm not letting the others seem me like this."
"They won't... You're going home as Asriel." Her hand over her heart in an odd way, as if implying some connection to what she was saying.
"What are you talking ab-... No." Flowey stated figuring out what she might be implying, "Human souls don't work, they don't change me back into him, they turn me into that... thing. You know that, you were there. And you don't have the stomach to collect the other kind of soul."
The flower turned away from her, staring down into his reflection in the pond silently for a few minutes. His face contorting into the fanged nightmare it became when he lost control if his anger.
"Even if you did, you know my other half would hate you, forever, for resorting to that." He hissed, black eyes glimmering in the dark and face contorted into a fanged smile as he twisted to stare at her. "Or for getting yourself killed for us. We already lost one sister..."
"I know. A soul without magic doesn't work. And I don't plan on anybody dying." She stated, seeming to wince in pain from some unseen source. "Alphy and I found a way around that..."
"Way around... What did you do?"
She just grinned, "A mortal body can't contain a soul that has both inside it..." She explained, shaking. "I'm no more built for Magic than they are for strong Determination... Hopefully I won't need it long..."
"you... YOU IDIOT! What were you thinking? That'll tear you apart!" The flower snarled, a history lesson about seven humans dying to create the seal, the one recorded time their kind had ever used magic, flashing through his mind. "You'll die, or melt or dissolve or who knows what!"
She didn't answer at first, just dropped to her knees. Getting caught by her friend's vines before she could collapse fully. But that didn't stop her, as she summoned forth a bit of magic, enough to expose the presence of her own soul and Flowey's emptyness. Grabbing hold of her own she seemed to be trying to pull it apart. "Too much for one... but for both..." she trailed off straining.
"You idiot. You absolute idiot. Stop, before it's too late." the flower pleaded, trying to halt her movements with his vines, or drain her strength, do something. "You can't do this, you'll die. I... I don't want you to die."
"No. I won't." She struggled through gritted teeth, Resisting the vines Flowey was summoning to sap her strength and stop her. Resisting the pain of what she was doing to herself. "I won't let it. I refuse."
Finally, with one last tug, it broke, snapping clean in half. It should have crumbled away to nothing right there...
But it didn't.
Flowey could only watch transfixed, a mixture of confusion and horror filled his mind even if he was still numb to the feelings that should have come with such thoughts.
Letting go of one half, that immediately retreated inside her, she grabbed the stunned flower and shoved the other half towards him.
Asriel awoke with a start, one hand clutching his chest as his heart raced. Looking around it was dark, the light coming in from the cave mouth far above the flower patch had dimmed considerably.
It was night already? How long had he been out, the last thing he could remember was this morning when...
"FRISK!" he shouted, answered by a groan to his right. Looking over, he spotted her prone form. Scooping the teen up, he carried her back to the old house as fast as he could move. Hardly noticing his own stature or robes given his mind was elsewhere.
Save for a few vines on the walls to give him easy access to the structure he had kept it as it had been. Thank heavens the monsters had decided hauling the furniture down a flight of stairs and through the entire underground wasn't worth it. Laying frisk down in his mothers' chair and lighting the fire, he dashed off to one of the other rooms to grab a blanket.
Wrapping her up in it, and staying by her side. Asriel waited for her to wake up, praying she would.
At some point he must have fallen asleep again, as he was coaxed out of dreamland by the not unpleasant sensation of a hand scratching at a spot behind one of his ears.
"Morning sleepyhead." Came the voice that finally jolted him awake.
"You're okay..." He almost broke down in tears as he hugged his little sister tightly.
It took them two days to get out of the underground. A lot of it had fallen into disrepair over the years. By the time they reached the surface, everyone was waiting; apparently Alphy isn't anywhere near as good at keeping hair-brained experiments a secret as she used to be.
It was a good thing Frisk hadn't told Alphy everything. Everybody's, especially Asgore and Toriel's, reactions to just who she 'needed to save' were priceless.