I think it might be a love story with a dark and gritty edge. Maybe the characters face life-threatening situations or have to make tough choices that put their love to the test, all within the context of the osu world.
I'm not really familiar with the specific story behind the red X in osu. It might be tied to a scoring system, a level completion marker, or something else game-specific. You might want to check the game's documentation or forums for a more detailed explanation.
Black Zetsu's story in Naruto is quite the tale. He was an entity that had been around since ancient times. He had an in - depth knowledge of the ninja world's history and used this to his advantage. He was the driving force behind many of the conflicts in the Naruto series. For example, he caused a rift between Sasuke and Naruto at times. His true nature was only revealed towards the end of the series, which was a shock to many characters. He was a master of manipulation, using the desires and weaknesses of others to achieve his ultimate aim of bringing back Kaguya.
One key event was his deception of Madara. He made Madara think he was his ally while actually setting him up for Kaguya's revival. Another was his influence on Obito. He pushed Obito to do things that served his purpose. Also, his role in the Fourth Great Ninja War was significant as he was behind many of the unexpected turns in the war.
When Naruto absorbs Zetsu in the fanfiction, it could impact his mental state as well. Zetsu has been involved in many dark and devious plans. Naruto might have to struggle with the knowledge and influence of those. However, on the positive side, Naruto could use Zetsu's ability to communicate with the plants and gather intelligence from the natural world. This would expand his range of information sources and give him an edge in strategic planning during battles.
It could change his relationship with Sakura. Sakura might be a bit wary of Naruto at first because of the unknown effects of absorbing Zetsu. But as she sees that Naruto is still the same person at heart, she would gradually accept him again. With Kakashi, he might be more cautious and keep a closer eye on Naruto to make sure he doesn't lose control.
Since I don't know the 'oruthee real life story', it could be a story full of adventures, like Oruthee traveling to different places, experiencing various cultures, and having some life...