The Underworld could also be a setting. He might be training there to face the dark forces. The dangerous environment full of monsters and traps would provide him with ample challenges to improve his combat and survival skills.
Well, in a 'percy trains himself fanfiction', he could be training for a big battle. Maybe he's practicing his combat skills with his sword, working on his speed and agility in a secret forest glade. He might also be training his mind, learning new spells or strategies to outwit his enemies.
It could be that Percy's sense of honor led him to sacrifice himself for Zoe. If he had made a promise to protect her, he would uphold it no matter what. Also, Zoe might have been the key to solving a major mystery or threat. Percy, realizing this, chose to sacrifice himself so that Zoe could continue and complete whatever task was necessary. Additionally, his love for his friends and the greater community of demigods might have made him believe that Zoe's survival was more important than his own.
One possible motive could be extreme depression. If Percy has been through a lot of hardships in the story, like being ostracized or constantly failing in his goals, it could lead him to this drastic step.
Even though Naruto is training himself, in the fanfic, he could still have some indirect mentors. For example, memories of Jiraiya might guide him. Jiraiya's teachings and the books he left behind could be like silent mentors for Naruto as he trains. Another possibility is that the spirit of the Fourth Hokage, his father, could somehow influence his training, giving him the drive and some hidden knowledge.
Obviously Percy himself is the main character. There could also be his friends like Annabeth. She might be supporting him in his training, or even training with him. Grover could also be involved, perhaps using his nature - related powers to help Percy in some unique training exercises.
Well, in some fanfictions, Ash trains himself in a special training ground. He sets up various obstacles to enhance his agility and reflexes. He also practices his Pokemon - handling skills alone, trying to communicate better with his team. This helps him gain a deeper understanding of their abilities and how to bring out their full potential during battles.
Well, in most fanfictions where Percy trains in Tartarus, it often involves him facing extreme challenges. He might be learning to control new powers or just surviving the harsh environment. For example, he could be training to fight stronger monsters that lurk in Tartarus.
Obviously, Ash himself is the central character. His Pikachu is also often prominently featured, as it's his most iconic Pokemon. Sometimes, his other Pokemon like Charizard or Greninja might also play important roles in his self - training journey, either as partners in training or as sources of inspiration for him.
One possible reason could be extreme despair. Maybe he faced some overwhelming challenges or betrayals that made him feel there was no way out. For example, if he lost all his friends and felt completely alone in the world of the fanfic.
Well, perhaps Percy could be learning some new and powerful combat skills from Chaos. Maybe Chaos teaches him to manipulate chaos energy in unique ways for battle.