Yes, I refer to NiGHTS as a girl here, but there's a reason: I believe the two-personalities theory. Basically, NiGHTS has two halves, one male and one female. Helen met the female one. See the Wild Mass Guessing page on TVTropes for Nights into Dreams for more details and evidence.
This story takes place immediately after the first Memory Forest cutscene in Helen's dream.
"Helen!" NiGHTS cried, reaching out towards her, but the girl had already sunk out of sight, the dark hole in the ground closing over her.
Reala was laughing again. NiGHTS whirled around to face him.
"Where did you send her?" she demanded.
Reala chuckled. "I sent her right through the ground, out to the other side," he said. "You know what's down there."
NiGHTS's eyes widened and she glanced fervently back at the spot of grass Helen had been standing on moments before. "You didn't."
"Oh, but I did," said Reala. "And since Visitors can't fly on their own, she had nowhere to go but down, down into the black sea. And as you well know, no Visitor comes back from the black sea."
NiGHTS lashed out at Reala, but he caught her fist in his hand.
"Come, now," he said. "Don't tell me you actually became attached to that meddling little Visitor? You are so sentimental, NiGHTS."
NiGHTS tried to strike him again, and again he grabbed her hand. He spun her around, twisting her arms behind her back. NiGHTS let out a short shout of pain.
"You really are weak without a Visitor's Ideya to help you, aren't you?" Reala taunted. "Well, you haven't got anyone to help you now, and even if the Visitor were to survive the black sea, she'd never allow you to Dualize with her again, now that she knows that you are a Nightmaren like me."
"You and me… We're nothing alike," NiGHTS protested weakly.
Reala chuckled again and rose into the air, carrying NiGHTS along with him.
"What did you think to accomplish, running off like that?" he sneered. "Did you actually think that you would defeat Master Wizeman? All that your insolence has done is made him angry. And when he's angry, those responsible for his anger suffer."
A cage sprouted from the ground, and Reala shoved NiGHTS inside. She hit the floor not very gracefully and quickly got back to her feet, glaring up at Reala.
"Besides," the red Nightmaren continued, "even if you did somehow manage to defeat our master, you would gain nothing. You and I would be destroyed as well. Are you really prepared to die for these humans?"
NiGHTS's fists were clenched at her sides.
"To keep Helen's dreams, and the dreams of every other Visitor, safe from Nightmare… it will be worth dying."
Reala shook his head. "My dear sibling… You are so very, very foolish. I will return shortly with our escort. Try not to cause too much trouble while I'm gone…not that you can do much from that cage, anyway."
NiGHTS watched him fly away. Three more Nightmarens were winging their way through the trees in his wake. They had the keys to her prison, she knew, but she had no way of reaching them now, not without Helen…
NiGHTS sagged to her knees, her head hung low.
The sound of flapping outside her cage caused her to raise her head to see Owl hovering there.
"Oh, NiGHTS, this is terrible, simply terrible!" he was saying. "Where's Helen?"
"She's gone," NiGHTS's voice sounded flat, even to her. "Reala dropped her into the black sea."
"Oh, my…" Owl gasped. "Well, if she's alive, she'll be at the Dream Gate. I'll go wait there. You stay right where you are!"
"It's not like I have a choice," NiGHTS muttered as Owl flapped away. "Silly old bird… Even if she did somehow survive, she won't help me now. Not now…"
She could still see the look of horror on Helen's face and the sense of betrayal in her eyes as Reala revealed the truth.
She should have protested more; she should have insisted that she was indeed Helen's friend, that Reala was lying…
…but she was a Nightmaren, and she hadn't been able to deny it. Helen's fear had hurt her; it had reminded her why she kept her identity a secret at all. Nightmarens were cruel. Nightmarens were feared. Visitors wouldn't be friends with Nightmarens. No, it was better to leave Visitors uninformed. That way, she could have some fun.
But the look on Helen's face… How could she have been so stupid, to let Helen see her wearing her persona? She'd only put it on at all because… because…
"Good evening, NiGHTS."
"Reala. Come to lock me up again?"
"You wrong me, NiGHTS. You think I'm out to harm you? My actions are only out of concern for my little sibling."
"'Concern.' Yeah, right."
"We used to be on the same side; do you remember how nice it was then? No fighting, no problems… Now you won't even wear your persona. I'm hurt, NiGHTS. Won't you put it on, for old times' sake?"
"…all right. But this doesn't mean I'm coming back, Reala."
"You look wonderful. Well, I'll see you around."
"Wait, where are you going?"
"Why the funny mask?"
"Ha ha ha… Oh, it's nothing! Come on, let's go."
Reala. He'd been planning this all along. He didn't care about her; he didn't care about anything other than himself.
He was right. She was too sentimental. And that had doomed a child to chaos.
Not just a child. Her friend, Helen. Betrayed by a Nightmaren and consumed by the sea.
There was something wet on her face. NiGHTS reached up to wipe it away, but then she stopped.
Am I…crying? I never cry. Nightmarens don't cry.
It must be all the Dualizing I've been doing lately. Yeah, that's it; these are just leftover human emotions or something. After all, I don't get scared, and I definitely don't cry. But a human would be scared and sad, if she'd just been betrayed by a sibling and lost a friend and was locked up to await punishment by her creator…
NiGHTS whirled around. A face was peering up over the platform. It was a very familiar face.
"…Helen?" NiGHTS whispered, unable to believe her eyes.
The Visitor pulled herself up into the cage, her hands glowing as her Ideya allowed her to pass through the barrier.
"But…how?" NiGHTS asked.
Helen shook her head. "We'll talk later. We have to get you out before Reala comes back!"
NiGHTS gaped for a moment, and then she grinned.
"Right," she said, extending a hand. "Let's go!"
Helen hesitated for an instant, just enough for NiGHTS to barely notice, but then she resolutely grabbed NiGHTS's hand.
As they Dualized, NiGHTS felt Helen's Courage flowing through her. Now the cage was no hindrance to her, and they flew out in pursuit of the winged Nightmarens. They easily overtook the three overgrown birds, paralooping the keys from their grasp and destroying the cages in what felt like no time at all.
As the last cage fell to pieces, they heard the Nightmarens squawking as they flapped away through the trees, off to inform Reala that their prisoner had escaped.
"Let's get out of here," Helen said. Together, they took off.
They left the sunlight of the forest for the twilight of the Dream Gate, where they separated again. Helen sat on the edge of the fountain, her hands folded in her lap as she looked at NiGHTS, who was hovering in front of her. For a few moments, neither spoke.
"How did you—?" NiGHTS began.
"Are you really—?" Helen said at the same time.
They both stopped.
"I'll answer first," said NiGHTS. "You deserve an explanation…more than I do."
Helen nodded slowly. "So… You're a Nightmaren."
"I am," NiGHTS confirmed. "I am a first-level Nightmaren, the same class as Reala. I was created by Wizeman to enslave Visitors and steal their Ideya, to give him more power and control over the Dream Dimension. But that was a long time ago, Helen. I don't work for him anymore, I promise. Reala was lying about that. I don't have any proof, but… I just wanted to be your friend, that's all."
She fell silent, waiting for Helen to tell her to go away, that she couldn't trust a Nightmaren, or even get up and leave the dream world herself.
What she didn't expect was for Helen to shrug and say, "Okay, I believe you."
"You… You do?" said NiGHTS.
"Of course, I do!" said Helen. "You're my friend, NiGHTS. I'm not going to stop trusting my friends because someone like Reala says to."
NiGHTS was still unable to believe what she was hearing. "So… You still want to be my friend… even though I'm a Nightmaren? A creature of Nightmare, a creation of Wizeman?"
Helen nodded. "Uh-huh. You're nothing like the other Nightmarens, NiGHTS. You're my friend!"
"I… Thank you, Helen," said NiGHTS.
Helen smiled and stood up on the fountain wall, reaching out towards NiGHTS, who gratefully received the hug.
She's really my friend! My friend, Helen.
I'll miss her once I'm gone…
"Is something wrong?" Helen asked, looking up into NiGHTS's eyes.
"Who, me? I'm fine." NiGHTS shoved that sad thought away. "Tell me, how did you come back? I thought you were gone for good!"
"Oh, I fell into another dream world, and the boy there led me back to the door," Helen explained.
"Another world?" NiGHTS said, surprised. "But that shouldn't be possible."
"Owl said that, too," said Helen. "But really, the boy fell into my world first, back in the castle, so I'd already met him here once. I had to lead him back, too; he said he had a friend who needed help."
"A friend, huh?" said NiGHTS, chuckling.
Oh, what trouble has my other me gotten into now?
"I wonder if your dreams will intertwine again," she mused.
"I hope so," Helen said. "He was a nice boy."
"So, what do you say we see what else is in that forest of yours?" NiGHTS suggested.
"Sure," said Helen.