Naruto: Aizen's Shadow in Konoha

"The system?!"

The mechanical voice resonating in his mind sent a jolt through Aizen. Despite being a dedicated scientist in his previous life, he wasn't unfamiliar with these legendary "cheat codes" prevalent in anime and novels – essential tools for time-travel narratives.

[Yes, this system is the Great Villain System of Myriad Realms, specifically designed to cultivate the ultimate villain across all worlds!] The voice boomed with a hint of pride.

"Cultivate a villain? The strongest villain in the world?!" Aizen's eyes widened further.

Although both his past and present selves shared the name of a notorious villain... he'd always considered himself a good person. Sure, his previous life's biological experiments might have involved some unfortunate sacrifices. But could scientific pursuits be deemed villainous? This was for the advancement of humanity! Collateral damage was simply a regrettable but necessary cost in the grand scheme of things. Aizen nodded resolutely, a smug expression on his face.

Regardless of the system's villainous agenda, it would undoubtedly offer him immense assistance. In simpler terms, he was in. Survival in this world hinged on acquiring greater power.

"System, what functions do you offer? Do I earn points by battling monsters, or are there rewards for signing in?" Aizen inquired mentally, a glimmer of anticipation flickering in his mind.

The world of ninjas wasn't exactly forgiving. Especially considering the original Aizen's memories revealed Konohagakure to be a 32-year-old village! 'What a time to be alive! In just one year, the Second Shinobi World War would begin!'

Even the ever-composed Aizen couldn't help but curse under his breath.

Landing in the world of Naruto, he'd already managed to offend the Sarutobi clan, potentially the Senju clan by extension, and now had a looming Second Shinobi World War to contend with. Talk about bad luck!

Strengthening himself was paramount.

[Sorry, the host body has not yet become the host of this system, and any system functions will not be open to the host body!]

The system delivered the news with a tone of indifference.

"What?" Aizen sputtered. All this time, he wasn't a host, just a potential one?

He maintained his composure, however, and inquired calmly, "So, how do I become a Host?"

[Completion of specific achievements is required.] The system responded.

Aizen furrowed his brow. "Specific achievements? Can you be more specific?"

[Unfortunately, divulging such information is against protocol. You must rely on your own exploration.] The system droned on.

"Ugh, what's the point then? Just tell me an achievement to complete! This is useless!" Aizen took a few deep breaths to control his rising frustration. As a former prodigy, blessed with exceptional abilities and emotional control, he rarely lost his temper, especially after achieving renown. This system, however, was pushing his buttons.

The system's behavior was indeed outrageous.

[While system functions are currently inaccessible, a starter package will be provided to aid you in completing your host trials.]

Perhaps sensing Aizen's simmering frustration, the system offered an olive branch.

Hearing this, the furrow in Aizen's brow finally smoothed out.

[The starter package includes the complete memory inheritance of Shinigami Aizen Sosuke, along with his Zanpakutō – Kyoka Suigetsu! Distributing to the host now!] The system's voice softened slightly.

"Shinigami Aizen Sosuke's memories and Kyoka Suigetsu?!" Aizen's eyes gleamed with sudden excitement. The legendary captain's full power and his mesmerizing blade – this was a game-changer!

Just as he prepared to delve deeper with further questions, a sharp pain akin to absorbing the original owner's memories lanced through him again!

[Ding! A starter package was issued. The system will now enter a dormant state. If the host fails to complete the designated Host achievement task, the system will not resurface.]

As Aizen's head throbbed, the system's voice faded, growing weaker until it completely vanished.

"Damn system!" Aizen couldn't help but curse. Today marked his second outburst.

After a while, having fully absorbed the memories of Shinigami Aizen, Aizen leaned against the pole, catching his breath. Anyone unfamiliar with the situation might mistake him for a victim of a female bully.

"Aizen, are you alright? Are you worried about Sarutobi's threats?" A concerned voice broke the silence.

Aizen turned to see Hideki Kitagawa, a close friend from Konoha Aizen's memories. Kitagawa, eight years old like Aizen, was his classmate. Frail with short black hair and matching eyes, he possessed a somewhat delicate appearance.

"It's nothing, just a minor chakra issue," Aizen offered a reassuring smile, mirroring the original owner's gentle and kind demeanor.

He was essentially a squatter in this stolen life, a black-hearted imposter in a world of ninjas. Here, caution was paramount to avoid raising suspicion.

"Did the fight with the Sarutobi clan injure you? It's all my fault, Aizen. If it weren't for me, you wouldn't be hurt or targeted by those genin," Kitagawa expressed his guilt, deeply troubled by the situation.

Looking at the original owner's friend, a pang of guilt flickered in Aizen's chest. Yet, he maintained his gentle facade, replying, "Don't worry, Hideki-kun. As a friend, this was the least I could do. There's no need to burden yourself."

His words deeply moved Hideki, stirring a wellspring of gratitude within him.

"But the trouble with the Sarutobi clan..." Hideki started cautiously.

"No worries, I have a plan to deal with them. Don't fret about it. I have something to attend to and need to head home soon. You should too!" Aizen cut him off, eager to leave the watchful eyes. He craved solitude to test the power of Kyoka Suigetsu, now a part of his being.