Help Needed (Quickly, please!)

I'm pretty sure it was revealed in the commentary.

It's never said in the movie, and the guy never shows up again.

It's Uesugi Kenshin, the two's legends are basically inseparable so much their rivalty is renowned. In link with Chinese mythology, Shingen was dubbed "Tiger of Kai" and Kenshin "Dragon of Echigo" by their contemporaries because their rivalty was relentless, and also because the two were widely known to respect one another with a lot of chivalry.

Kurosawa has placed Kenshin as a big contrast. Shingen and Kenshin has been rivals for almost two decades, to the point of actually fighing each other mano-a-mano at the Fourth Battle of Kawanakajima, yet he respected him so much as a person that he would mourn him as he had lost a brother or a childhood friend. Compare with the two "newcomers" with novel ways, to Ieyasu's admirating regret that "we have lost a great leader", almost like a learner losing a mentor, and to Nobunaga's pragmatic concern that maybe the most threatening warlord to his rule was now gone from the scene, with no mourning whatsoever. In fact, later Nobunaga actually celebrates his death by performing his favourite part of Atsumori.