In Part 4 of JoJo's Bizarre Adventure, we are introduced to Rohan Kishibe, who has the ability Heaven's Door. Heaven's Door turns people into books that detail their lives, which Rohan can read or write on to control those people.
We first see this with Koichi, who is directed to never attack Rohan when they first meet and who is made to forget what happened.
Later, when they are walking through an alley and Koichi turns back, despite warnings not to do so, Rohan saves him by using Heaven's Door to write that Koichi will be flung past the boundary of the alley.
In the side story Thus spoke Rohan Kishibe, we also see Rohan use Heaven's Door on octopi to get them to save him and Tonio when the two are drowning.
What are the limits of this ability? I assumed that it was a question of what is physically or logically possible, rather like the limitations of Death Notes in Death Note.
However, I just saw the beginning of Part 5, where Koichi implies his fluency in Italian is to Rohan's use of Heaven's Door. This seems less plausible as something that could happen organically, when Koichi seems to have gone to Italy on short notice and when language acquisition takes a lot of time. While it could be interpreted as similar to memory manipulation, it's one thing for Rohan to take away existing memories, and another for him to add in something that never existed.
