One common theme is the power dynamic. Since Geto has strong powers in the Jujutsu Kaisen universe, exploring how that affects his relationship with the reader can be interesting. For example, he might protect the reader using his powers.
Sure. One popular 'kazuki x yuuji fanfic' might be 'The Bond of Kazuki and Yuuji' which focuses on their friendship and the adventures they go through together. It beautifully depicts their relationship growth.
Another option is to check out Tumblr. There are often fanfic recs and some users post their own Geto x Reader fanfics. Follow some relevant tags like #Geto x Reader or #Jujutsu Kaisen fanfic to find new and interesting stories.
Since I'm not familiar with a specific 'Passion Yuuji' novel, in general, a novel might have unique characters. Maybe the characters in this novel have strong emotions and passions that drive the plot forward.
I'm not sure as I haven't played this visual novel specifically. There could be a lot of different interactions between characters in a visual novel, but without experiencing it first - hand, I can't say for certain.
He might be depicted as confident. In a visual novel, this could be shown through his interactions with other characters, where he takes the lead in conversations or situations.
In many Geto and Gojo fanfictions, the theme of trust is crucial. Their trust in each other can be tested in various ways. Another common theme is the exploration of their past. What made them who they are? How did their past experiences shape their relationship? And the theme of growth, both as individuals and in their relationship with each other, is also often seen.
His relationships, especially with other characters like Gojo, are often explored. Fanfics might show the deep friendship they had before Geto's fall and how it deteriorated. It gives more depth to their dynamic.