Master’s degree to apply for a PhD?

This is way into the future, maybe 10 or 15 years from now that I'm even going to attempt to get in, but I really want a PhD in Literature from a University in Japan, namely Keio. (I know, aiming pretty high, right?) I was just wondering when I apply for the program, if I'll have to have a master's first.
Also, if I apply for the master's at Keio (or any other university), get in and complete the program, would my chances of getting into its doctoral program higher?

Your best source of information for answering your questions is the university you are interested in. They are the only ones that can definitively answer what their requirements are.

However, here are some general requirements to consider. First most universities require you to have a Masters before you can apply to a PhD program. However, there are some universities that will allow you jump into a PhD program from within a Masters program before you have completed all of the Masters requirements. But most likely in order to do this you will have to have amazing grades and pretty much blow them away with your Masters research. Second thing to consider, universities for the most part don't recommend completing both your Masters and your PhD at the same university. So you wouldn't do your Masters at Keio and then do your PhD as well. It is generally frowned upon. They want you to branch out and network so most people (this is not true in all cases, there are always exceptions) get there Masters at one university and then do their PhD at a different university.

In answer to your question about your chances being higher if you had a Masters. Obviously yes. No university is going to consider you for entry to a PhD program with a Masters.

That last line should read "without a Masters".