Further context: First European contact with Japan was likely around 1540

http://en.wikipedia.org/wiki/Chronology_of_European_exploration_of_Asia

Shows that Keanu's character of a "half-breed" is at least historically possible, given the 47 Ronin legend occured ~160 years after first contact.

To give some perspective, we are currently living in a period ~160 years after first meaningful, significant Yup'ik-European contact, since the silty river deltas of the Yukon and Kuskokwim were avoided by European sailors for a long time due to their difficulty of navigation.