Jiraiya is another person. Since Jiraiya trained Naruto and was like a father figure to him. Naruto could write about their travels together, the lessons Jiraiya taught him, and how much he misses Jiraiya after his death. He might also mention the pervy things Jiraiya did that always made him laugh.
His teammates, Sasuke and Sakura, would be frequently mentioned. Naruto would write about their missions together, the arguments they had, and the growth they all experienced. He'd probably talk about Sasuke's cool jutsus and Sakura's increasing strength in medical ninjutsu.
Maybe Naruto uses the diary to talk about his relationships. He could write about his friendship with Sasuke, how much he misses him when he's away, and his determination to bring him back. He might also write about his feelings for Sakura, like how he's trying to become someone she can rely on.
Jane is likely a fan - created character. She could be an ordinary girl who somehow gets transported into the Naruto universe and starts writing a diary about her new life there.
His friends like Sakura, who is a medic - ninja, would surely be the first ones to try to help. She could use her medical knowledge and skills to diagnose Naruto's illness.
It could be a rogue ninja from a hidden village. There are always some ninjas who go against their village's rules and become outlaws. They might target Naruto because of his reputation or the power of the Nine - Tails within him.
Maybe a rogue ninja group that wants to sell Naruto to the highest bidder. They see him as a valuable asset due to his strength and status in Konoha.
Naruto could meet some young and upcoming Iwa ninja trainees. They might be curious about Naruto and his abilities from Konoha. These trainees could either look up to Naruto or be jealous of his fame, leading to various interactions. Also, there could be some old - timer Iwa ninjas who remember past events involving Konoha and have their own views on Naruto's presence in Iwa.
There's a possibility that Naruto meets some of the younger generation of shinobi in Suna. These new characters could be eager to learn from Naruto or challenge him to a friendly spar. He may also come across some of the elders in Suna who have their own stories and wisdom to share with Naruto.
It could be other villages as well. Since Naruto has made friends and allies from different villages during his adventures. For example, the Sand Village might send some of their ninja to help, considering Naruto's relationship with Gaara.
He might meet his sensei, Jiraiya. They could have long talks about Naruto's growth as a ninja and Jiraiya could give him more wisdom from the afterlife perspective.
It could be Orochimaru. He's always been interested in power and experimenting. Maybe he sees Naruto as a perfect guinea pig to test some new jutsu that goes horribly wrong and causes Naruto to explode.