WARNING: UNMARKED SPOILERS FOR THE LIGHT NOVELS AND MANGA!
The premise itself: one girl is reincarnated as the final boss of an RPG/otome game. Rather than get killed by the heroine and her party of love interests, she decides to level grind early so that she stands a chance against them. She proceeds to do so for the next 10 years, starting at the age of five, entering dangerous dungeons by herself with no protection charms and mowing down so many monsters that they recognize her on sight whenever she returns to said dungeons. By the time she enters the academy, her strength is near Superman levels of absurd and her dark magic is powerful enough to create black holes. Naturally, when she finally gets her level tested, she's at Level 99, which is unheard of in her world.
With the threat of expulsion by Prince Edwin looming, Yumiella is given permission to go all out with her magic. She obliges by launching her most powerful spell, Black Hole, into the sky. Said spell produces a black sphere that sucks away all the air within it, and when it pops, produces dangerous winds that replace the air that was destroyed. It was so big that it could be seen from miles away, leaving no doubt from anyone that Yumiella is Level 99.
The best part: she was still holding back. When Yumiella takes her turn for the school tournament, she unleashes a Black Hole spell using all of her power. For a moment, the black hole is so huge that a light brighter than light magic shines through the stadium. And then typhoon-level winds come blowing through, nearly destroying the area.
Yumiella is able to heal injuries like light magic does. She can even reattach limbs that have fallen off! Unfortunately, it's not pretty to watch as it bubbles and churns as it heals.
Thanks to all of her level grinding, Yumiella's senses have heightened. That means she can hear a whispered conversation as if it was a normal volume, and see farther than most human eyes can.
Yumiella not only swatting a dragon out a classmate's family's home, but getting to keep a dragon egg as a reward. Dragons are notoriously difficult to tame, as you need to pour as much magic as you can into the egg so that the dragon will recognize you as its mother, or else risk having it go wild. Yumiella's Level 99 magic is so unrestrained that Ryuu becomes a lovable puppy of a dragon that's smothered in love by Yumiella, and is second only to her in power.
Patrick revealing he, too, has been level grinding in order to meet Yumiella's (fake) requirements for a fiancé. When Yumiella finds out, he has achieved Level 60, qualifying him for the Demon Lord elimination party.
He eventually reaches Level 99, an event that makes Yumiella giddy with joy as she finally has an equal who can stand toe-to-toe with her.
Unlike her light novel counterpart, Alicia occasionally slips into a trance-like state that forces her to do things that further the original game's plot, like enrolling in the Academy or leveling up, against her will. This unfortunately includes stabbing Yumiella in the back after the Demon Lord's defeat, since she's supposed to fight her in the original game's ending. With help from the three harem boys, Yumiella calls out to Alicia and helps her break through the trance, using only The Power of Friendship and Edwin's declaration of love to do so.
The fact that Yumiella decides not to defect to another country, instead taking her parents' titles for herself and working together with the rest of the Dolkness county to revitalize the territory and prove that black-haired people and dark magic users are not as bad as they seem. She manages to succeed in changing the minds and hearts of the people in her territory, though maybe not in the way she intended.
Credit also has to go to Daemon, the deputy of the Dolkness county, who managed to keep the county afloat for twenty years while the Dolkness parents sucked all the money away for their own pleasures. He's quite understandably relieved when Yumiella shows she's willing to be more involved in the county's development than her predecessor.
Yumiella finally meets her match in the magic barrier protecting the Samonistic church in the royal capital. Said barrier is so strong that it breaks Yumiella's wrist clean in half, and takes multiple hits from her black magic to finally shatter it.
Her match with Yumiella 2 is so even that it ends both times in an unofficial tie. Both of them are too alike in power and strength to properly overpower one another.
Same with her battle agains the God of Evil. His "administrator" powers are able to freeze people in place, and any hits from Yumiella don't do any damage. Only through Yumiella 2's sacrifice, the power of the "Keep Living" sword, and her accumulated experience after reaching Level 99 early allow Yumiella a chance to beat him for good.
The "Keep Living" sword has the power to create dimensional rifts. Using that, her now invincible magic, and the elixirs capable of bringing things "back to life", Yumiella is able to turn back time on Yumiella 2's dimension to one year prior, bringing humanity (and Yumiella 2) back from the dead, a feat thought impossible by the gods.
