Maybe Tsunade. She has a lot of knowledge and power. She was also close to Jiraiya, so she might feel a sense of responsibility to guide Naruto in the fanfiction. She could teach him about different aspects of chakra control and some advanced medical ninjutsu that could be useful in battle. Also, she could offer him advice on leadership, as she is the Hokage.
There's a possibility that an unexpected character could become his mentor. For instance, an older and wiser shinobi from a different village who has heard of Naruto's story and is impressed by his resilience. This character could bring in a whole new perspective and set of skills that Naruto has never been exposed to before. It would add an interesting twist to the fanfiction, and help Naruto grow in ways that are different from what Jiraiya had taught him.
Kakashi could step up more as a mentor. He already has a connection with Naruto and could help him develop his skills further.
In fanfiction, Jiraiya's death can also be used to explore themes of sacrifice. He sacrificed himself for the greater good, and this can be further elaborated in different stories. For example, some fanfictions might show how his death inspired other characters to be more selfless.
Naruto is devastated. He feels a huge void in his heart. Jiraiya was the one who believed in him from the start, and his death makes Naruto question a lot of things. He might even start to doubt his own strength for a while.
Sometimes Naruto copes by throwing himself into his training. He uses the pain of losing Jiraiya as fuel to become stronger.
One scenario could be that Jiraiya is poisoned before his fight with Pain. In the fanfiction, an enemy within the village who is secretly working for Pain's forces might have poisoned his food or drink. So, when he goes to face Pain, he is already weakened, and this ultimately leads to his death. Another possibility is that Jiraiya's death is a result of a time - traveling plot. If there is time - travel involved in the fanfiction, his actions in the past might accidentally create a situation where he has to die to preserve the timeline.
He might become more determined. Losing Jiraiya was a huge blow, but it could fuel his desire to become stronger and protect the village even more.
His character may become more withdrawn at first. Losing Jiraiya, who was like a father figure to him, could make Naruto less open and more focused on his internal pain.
At first, Naruto might isolate himself. He doesn't want to be around others as he is so consumed by his grief. He just sits in his room or goes to a quiet place where he can cry alone and think about Jiraiya.
The training makes Naruto more confident in his abilities. Jiraiya constantly challenges Naruto and as Naruto overcomes these challenges, he realizes his own strength. He starts to believe in himself more, which is evident in the way he faces stronger opponents without hesitation later on. Also, Jiraiya's teachings about the ninja way and the importance of protecting those you love make Naruto more determined and focused on his goals.
In this fanfiction, Kakashi could also be Naruto's mentor in magecraft. Kakashi is known for his adaptability. He could start to study mage powers along with Naruto. They could learn together about new forms of magic, and Kakashi could use his combat experience to help Naruto understand how to use his mage powers in battle situations.
Naruto might also isolate himself in certain fanfic scenarios. He feels that no one can truly understand the depth of his grief. He spends time alone, reflecting on all the things Jiraiya taught him and the memories they shared, like their time training on that toad -infested island.