In fanfiction, Percy can be really different after Tartarus. He might have a darker edge to his personality. The things he witnessed and endured in Tartarus could have toughened him up in a way that makes him seem colder on the outside. His fighting style could also be different. Maybe he's more brutal or efficient, having learned from the battles in Tartarus. His connection to the ocean could also be altered, perhaps stronger in some ways or more unpredictable due to the trauma he's been through.
In some fanfictions, Percy might be more withdrawn. He has seen the horrors of Tartarus and it could make him less trusting of the world around him. He may also have new powers or a deeper understanding of his abilities that he gained from the experiences there.
Well, in a lot of fanfiction, Percy after Tartarus is often depicted as being more haunted. He could have nightmares regularly. His relationships with others might change too. For example, he may be more protective of Annabeth because of what they went through together in Tartarus. Also, his view of the gods could be even more jaded as he's seen the true depths of their realm in Tartarus.
Look to Chapter 1 for the clearest answer. It begins with Jiang Li, I'm warning you, don't set your sights on Yinwan anymore, or..., keeps building through Following a bout of tinnitus, she struggled to open her eyes and finally..., and lands on Yet before she had the chance to teach her a lesson.
The best chapter pointer here is Chapter 4, "She Can Actually Tell Fortunes?!". It opens with Qiao regretted her words as soon as they left her mouth. and closes on A little girl acting mysterious, learning fortune-telling, what is this....
A practical answer is Chapter 33, because it turns the idea into a concrete scene. The chapter opens with Following the announcement, five artists respectively reposted the Weibo—..., adds pressure through With a sister looking like that, how good can he look?], and ends on Although Su Yinwan was.
Chapter 926, "Doomsday Beehive Exploded?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with What's this?" Zhao Lang looked at the unimpressive box in his hand...
Chapter 24 is the strongest chapter-level answer because it starts from Not only that, even the non-mainstream photos she had previously posted on... and follows through on Shi Xu rarely feels like going out, so that's nice." Aware that he....
Chapter 850, "Wanted to Recognize Her as a God-Granddaughter", is the clearest early answer because it turns Rong Junsheng's situation into a concrete plot event. The chapter gives readers a real answer by opening on Though Jiang Li might guess her family situation, he would never know...
Chapter 874, "All the Same", is the clearest early answer because it turns Lu Yafei's situation into a concrete plot event.
Chapter 13 is the strongest chapter-level answer because it starts from Sensing that something was off with Jiang Yan, Jiang Yan dismissed the... and follows through on How could this happen..." Lin Manru didn't want to hear any more of her....
If you want the clearest chapter reference, go to Chapter 17. Early on, Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and...; later, the chapter leans on Okay, let's go eat." After speaking, she strode forward..