Chrome Shelled Regios - Volume 20, Mortal Technica

After choking to death on a watermelon, Rong Jiahui thought that she would enter the netherworld and be reincarnated.

But when she opened her eyes, she discovered, to her very horror, that she had returned twelve years into the past…

Looking at the beautiful and awkward “little brother” that would be conferred the title of Marshal in the future for outstanding military service, she almost let out a snort of laughter.

She firmly decided to hug the thigh of this little brother, and climb to the top of her life!

The plan was beautiful, but it had just one problem.

Why had this little brother… suddenly turned into…

A little sister…

- Description from Novelupdates

The day dungeons appeared on Earth, everything changed! From ordinary beasts to monstrous ones, attacks were common throughout the globe, and no one was safe.

After countless studies using these beasts as subjects, Humankind developed a serum capable of awakening incredible abilities in humans, leveling the battlefield.

With the serum came heroes and villains, those who wished to use their powers for good or bad.

The Garmony Upper School was a place for the elite with the sole purpose of creating heroes to fight the villains and explore more of the dungeons after the beast attacks increased for no apparent reason.

Damian Romero, a youth who received the serum after turning 18, yet got no unique ability. It was not uncommon for one to not receive power, but it was still a blow to one's pride.

He went rogue after the news, drinking alcohol and having a good time with women, and it was at that moment, when he was putting his toy to play, he got a surprise.

[All-in-One System Unlocked]

Damian's unique ability was a system with multiple functionalities, but after he studied it, it seemed this system worked with a different type of coin called Erotic Coins!

Improve his body? Buy any item or unique ability from his system's store? All Damian needed was Erotic Coins, and he knew how to earn them!

Within the Immortal Realm, there were nine thousand major realms, with three thousand at the top, three thousand in the middle, and three thousand at the bottom, along with countless minor realms.

Due to a problem in his soul, Rean never cared about anything in his life. Even though he died heroically, there wasn’t really a 'will' behind that action.