Can Keima know all about your character/stereotype after learning about a bit of their plot or key characteristics?
It's only necessary if your character is a visual novel/dating sim character, but I'm opening it to all anyway just in case.
Being a very avid gamer, Katsuragi Keima can sometimes be too genre savvy. Fortunately, the details are often not specific, such as how games sometimes explains how dungeons are formed or how magic works. The problem comes if your character is a stereotype or a visual novel/dating sim character. With enough details of your character plot in canon, he can figure out your character's full personality and sometimes, the development of future events if it mimics something that happens in the game's canon. He often uses it to his advantage by manipulating your character's reactions if they are key to reaching his goal. Just in case I don't know your canon though, It would be nice to mention some canon details that may come to Keima's attention if you say yes.
There's not much danger if your character isn't a visual novel/dating sim character, but I could work something out if you want him to know your character's canon anyway. It's especially easier if your character is from a video game. He'd play just about any game as long as it involves making a girl in it happy, so all that's needed is a plot/gameplay tweak that'll make the focus making a heroine happy. For others, there's probably a game similar to the story of your canon that Keima played before.