Who he was
Evidence grade: Traditional · Later accountsArima Kihei is known from two early sources. Musashi's own book calls him a swordsman of the Shintō-ryū and says Musashi beat him at thirteen. The 1654 monument repeats this. The monument is not a second witness, because it follows Musashi's own wording.
The school is the Shintō-ryū of Tsukahara Bokuden's tradition. It is not the Kashima Shintō-ryū, a mistake that appears in some books.
Nothing independent confirms that Arima existed. His age, face, home and rank are not recorded. A 1727 book says he was a travelling fighter who had come to Harima. A book of 1755 calls him a Harima man. A book of 1776 says he was said to belong to the family of the Arima school. That last claim has no support and is graded legend.
The film draws him as a man of about 35 to 40 with a neat topknot. That age is a design choice made for the film. No likeness exists.
The notice
Evidence grade: Traditional · Later accountsAlmost everything about Arima's behaviour comes from one book, the Tanji Hōkin Hikki, written in 1727. It says he set up a fenced ground on a beach in Harima and put up a gold-polished notice board. The board said he would fight anyone who wished.
On the day of the bout, the same account says, he changed his clothes, put on loose trousers called karusan, and sat on a camp stool.
When the boy blacked out the board and wrote his own challenge, Arima sent a messenger to accept. Then the boy's uncle came to say it was a child's mischief. In this account Arima answers that he has nothing more to demand, but that his notice was blacked out and he cannot excuse himself to people. He asks for a public apology at the ground the next day.
So in this version he is not cruel. He offers the family a way out. No money motive appears.
No example of a notice board like this has been found for the 1590s, though such boards were plausible.
The bout
Evidence grade: Traditional · Later accountsThe 1727 account of the bout is short. The boy comes in leaning on a stick six or seven shaku long, about two metres, taken from the firewood. He carries only a short sword. He runs at Arima and strikes.
Arima stands up from his stool and cuts with a draw-stroke. They are too close for the blade to wound either of them, and their arms come to rest on each other's shoulders. The boy drops the stick, ducks under, lifts Arima onto his shoulder and throws him head first onto the ground. He picks up the stick and strikes fourteen or fifteen times. According to the book, Arima dies at once. The crowd is amazed.
Other tellings differ. A town website in Sayo says Musashi challenged Arima at a bridge and felled him with one blow. A museum retelling has the boy lifted and slammed, then beaten to death.
The film follows the 1727 order of events. Every movement between those points, and where the blows land, is in no source. The film invented them.
How we know
Evidence grade: Traditional · Later accountsMusashi's own words about this fight are the shortest version. At thirteen he fought Arima Kihei and beat him. The Japanese word means "beat", not "killed".
The 1654 monument says Musashi won "at once". It names no blows and does not say Arima died.
The first text to say Arima died is the 1727 book. That is why the film says "later accounts say Arima never got up".
The year is also uncertain. 1596 is a calculation from Musashi's age. A birth in 1582 would put the fight two years earlier. A fair window is about 1594 to 1597.
The place drifts. The 1727 book says a beach in Harima. Modern local tradition places it at a bridge in Hirafuku, a town in the hills. The earliest mention of Hirafuku the project found is a local history from 1926, and the research site it was read on calls the Hirafuku site a late legend with no evidence. The beach and the bridge cannot both be right.