Oh my bad then, I got it confused since Sugawara-no-Michizane was mentioned at some point. My thought process was he's either a monk or Sugawara-no-Michizane. He can't be kukai since we've already met him and my mind just went to sugawara I guess because nasa mentioned him? I'm sure we'll meet him at some point as well
We've had a LOT of historical figures, mostly from Japan, mentioned in this manga, to be honest. Sugawara no Michizane got mentioned back during the Kaguya-Emperor romance backstory arc that started with
Ch.197 and ended with
Ch.207. Specifically,
Ch.204 is when Michizane first shows up and is mentioned in the manga, far as I'm aware, seeming to heavily imply he originally wrote the Tale of the Bamboo Cutter based on the passed-on, barely-recalled rumors that Tsukasa overheard 300 years prior while she was still mortal and working as an apothecary through to her suffering from the illness that nearly claimed her life.
The only other time I recall Michizane coming up was when the next Japanese figure shows up:
Taira no Masakado in
Ch.216, during the arc about Nasa's grandfather. The same chapter, another figure is mentioned,
Tokugawa Ieyasu. There were translation notes at the end of the chapter explaining the connections and historical figures more clearly, too.
And another Japanese figure was covered in
Ch.181,
Shizuka Gozen. Heavily implied to have reincarnated as the 'headphones schoolgirl', Kurenai Hotaru. In connection to Shizuka was also mention of
Minamoto no Yoshitsune.
Before these arcs, there was also mention of a French inventor,
Nicholas-Joseph Cugnot, back around when the story first talked about Nasa getting a driver's license (
Ch.161).
Before that was Tsukasa calling someone named 'Ike' from a US base in Japan back during the big Tokiko flashback arc- I feel like this was basically saying she knew
President Dwight "Ike" Eisenhower and was literally calling in a favor from the President to get her and Tokiko into the States.
And, of course, we should all know the several references to
Prince Shotoku throughout the story, with at least a few instances implying Nasa looks like him and reminds Tsukasa of him, as well as that sequence of Nasa somehow recalling a memory of Shotoku's, leading to the idea that Nasa might be a reincarnation of Shotoku much like Kurenai seems to be a reincarnation of Shizuka.