*Location — GDC*

In the bustling confines of the Millennium Science School's Game Development Department, the familiar sounds of sibling rivalry echoed through the room. Momoi and Midori Saiba, the energetic twin sisters, were once again embroiled in a heated debate over their latest gaming conquest.

"I told you, I won fair and square!" Momoi declared, her pink hair swaying as she gestured emphatically.

Midori, the more reserved of the duo, adjusted her glasses and retorted, "You've only won once, and that was by sheer luck. I've beaten you three times already."

Observing the escalating dispute, Arisu Tendou sighed softly. As a fellow member of the Game Development Department, she was accustomed to the twins' spirited disagreements. "Let's all take a deep breath and calm down," she suggested, her voice carrying a hint of weariness.

From behind a slightly ajar locker door, Yuzu shyly peeked out, her curiosity piqued by the commotion. "Is everything okay?" she inquired timidly.

Arisu offered a reassuring smile. "Just the usual sibling squabble. Nothing to worry about."

Before the conversation could continue, Arisu's phone buzzed insistently. Glancing at the caller ID, she saw Rio's name displayed. Answering the call, she greeted, "Rio, what's up?"

Rio's voice, calm and composed as always, conveyed a sense of urgency. "We have an important mission that requires immediate attention."

Arisu's eyes lit up with excitement. "A quest? Count me in!"

The twins paused their argument, turning their attention to Arisu. "Did you say quest?" Momoi asked, her competitive spirit reignited.

Midori nodded in agreement. "We're ready. What's the mission?"

Yuzu, stepping out from her hiding spot, added softly, "Yes, please tell us more."

Rio's voice resonated through the speaker. "Millennium has partnered with Abydos to investigate incidents that pose a threat to Kivotos. We've received reports of a large crater in Millennium territory and unusual fluctuations in the halo rings above. Your task is to gather information on these anomalies and identify any potential threats."

Arisu's expression grew serious. "Understood. We'll head to the crater immediately."

As the call ended, Momoi pumped her fist into the air. "Adventure awaits!"

Midori, though more cautious, couldn't hide her curiosity. "We should proceed carefully. Something about this feels... off."

Yuzu nodded, her apprehension evident. "I have a bad feeling about this."

Despite their reservations, the team set out towards the reported crater. Upon arrival, they were taken aback by the sheer size of the depression in the earth.

"This is massive," Momoi whispered, her earlier enthusiasm giving way to awe.

Midori surveyed the area, her analytical mind racing. "What could have caused this?"

Before anyone could speculate further, a figure emerged from the shadows. A girl with striking white hair interlaced with black strands, piercing red eyes, and a distinctive black hoodie tied around her waist. Her attire was completed by ripped white jeans and black heels, and she wielded an imposing AR475GAR firearm. Most unsettling was her halo—a cracked star with a hollow eye at its center, emitting a sinister dark red glow.

The mysterious girl spoke softly, her gaze fixed on Arisu. "Target acquired: Kei."

Arisu's heart skipped a beat. The name 'Kei' resonated deeply within her. Memories of a past life, a previous identity, flooded her mind. But Kei was gone. She was Arisu now.

Momoi stepped forward, her protective instincts kicking in. "Who are you? What do you want with Arisu?"

The enigmatic girl tilted her head slightly. "Reinforcements detected. Engaging."

Without warning, she lunged forward with inhuman speed. Before Momoi could react, she was slammed against the crater's wall, the impact rendering her unconscious.

"Momoi!" Midori screamed, rushing to her sister's aid.

Yuzu, trembling, tried to muster the courage to intervene, but the assailant was relentless. She grabbed Yuzu by the collar and threw her to the ground, knocking the wind out of her.

Arisu's mind raced. She couldn't let her friends suffer. Drawing her railgun, she aimed at the attacker. "Stay back!"

The girl smirked, raising her own weapon. In a split second, she fired, the bullet striking Arisu's railgun and causing it to explode. The force of the blast sent Arisu sprawling.

As she struggled to her feet, the mysterious girl advanced, her odachi gleaming menacingly. With a swift motion, she impaled Momoi, who had been trying to rise.

"No!" Arisu's voice broke, a mixture of anguish and fury.

A voice echoed in her mind, a whisper from the depths of her consciousness. "Burn it down. Burn it all down."

An intense blue flame erupted around Arisu, enveloping her in a protective aura. When the flames subsided, she stood transformed. Her attire had changed

Arisu's transformation was nothing short of extraordinary. Her attire had shifted to a dark blue dress, and her eyes now gleamed with dual hues of magenta and blue, symbolizing the intertwined consciousness of both Arisu and Kei. In her grasp materialized a formidable sword, its blade a fusion of blue and black, while above her, a halo shimmered, merging the distinct colors of both personas.

The enigmatic assailant, undeterred, lunged forward, her Odachi clashing violently against Arisu's newly formed blade. Sparks erupted from the point of contact, illuminating the tension between the two.

High above the crater's edge, Rio observed the unfolding battle with a mix of astonishment and concern. Her earlier hypothesis about the surge of energy leading to this confrontation had been accurate, but the reality of the situation was far more complex than she had anticipated.

Descending swiftly, Rio approached the injured Momoi, who was struggling to her feet. "Momoi, what happened here?" Rio inquired, her voice laced with urgency.

Momoi winced, clutching her side. "We received a mission directive... from you. But when we arrived, she attacked us," she explained, nodding towards the mysterious girl now engaged with Arisu.

Rio's brow furrowed in confusion. "I never issued such an order. This was a setup," she stated, her gaze shifting to the fierce duel before them.

As the battle intensified, Arisu's movements became a seamless blend of her own agility and Kei's combat prowess. Each strike and parry was executed with precision, reflecting the combined skills of both entities. The mysterious assailant, though formidable, began to show signs of frustration, her attacks growing more erratic under the relentless defense.

Rio's eyes narrowed as she analyzed Arisu's transformed state. "This fusion... it's as if Kei has manifested through Arisu. But Kei is gone. How is this possible?" she murmured, her analytical mind racing to comprehend the phenomenon.

Momoi, still catching her breath, glanced at Rio with a mix of confusion and concern. "What do you mean? What's happening to Arisu?"

Before Rio could respond, a sudden surge of energy emanated from the battlefield. Arisu, drawing upon the combined strength of her dual consciousness, unleashed a powerful strike that sent the assailant skidding backward, her Odachi barely deflecting the blow.

The mysterious girl, now visibly agitated, glared at Arisu. "This isn't over," she hissed, before vanishing into the shadows, leaving the crater in an eerie silence.

Breathing heavily, Arisu's form began to revert, the blue flames dissipating as her attire and eyes returned to normal. She swayed slightly, the toll of the transformation evident, but remained standing.

Rio approached cautiously, her expression a mixture of relief and curiosity. "Arisu, are you alright?"

Arisu nodded slowly, her voice tinged with exhaustion. "I... I think so. But Kei... I felt her. She was with me."

Rio placed a reassuring hand on her shoulder. "We'll figure this out together. For now, let's get you and the others back to safety."

As the group regrouped, the weight of the encounter hung heavily over them. Questions swirled in their minds about the true nature of Arisu's transformation, the identity of their assailant, and the implications of Kei's apparent resurgence.