Jubai (Ken Watanabe) is seen speaking Ainu as well as Japanese in the film. The Ainu are the native people of Hokkaido, Sakhalin, and the Kurils located in Northern Japan. During the Meiji Restoration (which is mentioned in the film's prologue) the Ainu were assimilated into the Japanese population and their lands taken by the Japanese government, thus denying their indigenous status.
When Kingo Baba (portrayed by Akira Emoto) first speaks to Jubai Kamata (Ken Watanabe), he claims that Jubai is the one who 'killed Kondo is Satsuma'. This is a reference to the historical figure Kondo Isami; and leader of the Shogun's legendary Shinsengumi, who was indeed executed in the Satsuma Domain. Kondo was portrayed by Toshiro Mifune in the film 'Shinsengumi: Assassins of Honour' in 1969. By making this reference it establishes Watanabe's character as a force to be reckoned with in both historic and cinematic terms, as Kondo would be a recognisable figure throughout Japan.
The Satsuma Rebellion is referenced at various points in the film. This chapter of Japanese history saw disaffected samurai oppose the new imperialist regime of Japan. One of the most famous figures to come from this was Saigo Takamori. Coincidentally, Takamori was the inspiration for the character of Katsumoto, who Ken Watanabe portrayed in the 2003 film 'The Last Samurai', in which he starred alongside Tom Cruise and Hiroyuki Sanada.
Japanese remake of US film starring and directed by Clint Eastwood, Unforgiven (1992). Eastwood directed Ken Watanabe in Letters from Iwo Jima (2006)
Masaharu Kitaoji (portrayed by Jun Kunimura) is seen carrying two swords in the film. This was a samurai practice referred to as daisho, which literally translates into 'big-little'. The big blade was the longer and curved katana which would have generally been used for combat and duelling. The shorter blade was the tanto, which was a short sword most commonly associated with the ritual suicide seppuku (or Harakiri; meaning 'belly-cut').