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.
Well, it could be about Jane's experiences in the Naruto world. Maybe she's a new character who records her daily adventures, like her encounters with Naruto and his friends, and how she copes with the challenges of that world.
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.
Survival is also a big theme. Just like in the Future Diary anime where characters fight to survive in the deadly game, when combined with Naruto, it becomes about how the ninja from Naruto's world would handle such a life - or - death situation. They have to use their skills and wits not only against their enemies but also against the rules of the new, merged world.
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.
You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a wide variety of fanfictions, and you can use the search function to specifically look for Naruto Mary Jane ones. Just type in 'Naruto Mary Jane' in the search bar and browse through the results.
Some fanfictions might focus on a romantic relationship between Naruto and Mary Jane. In this case, it would explore how two characters from such different worlds could fall in love. It could show Naruto introducing Mary Jane to ramen and she introducing him to things like city life in her world.
Naruto has his chakra powers like the Rasengan and Shadow Clone Jutsu. Jane has the power to inflict pain on others just by looking at them. If they combined their abilities, Naruto could use his clones to surround an enemy while Jane uses her power. The clones could distract the enemy, and when the enemy is focused on the clones, Jane could strike with her pain - inducing stare. It would be a very effective combination in battle.
Well, there could be a fanfiction where Naruto meets Mary Jane during one of his travels outside of the ninja world. Mary Jane, being from a world with different technology and culture, could introduce Naruto to new things like modern music or photography. Naruto, in turn, could show her some of his ninja techniques, like the Shadow Clone Jutsu.
Naruto's boisterous personality would contrast with Mary Jane's more refined and sophisticated nature. However, they could complement each other well. Naruto could introduce Mary Jane to the concept of ramen, and she could introduce him to the world of Broadway shows. In a dangerous situation, Naruto would be the first to jump into action, using his speed and strength. Mary Jane, on the other hand, would use her intelligence and charm to help them out. For example, if they were trying to get information from a bad guy, Mary Jane could use her acting skills to deceive the person while Naruto stands guard, ready to attack if necessary.