For reference,
Shotoku was a leader of Japan, as well- he was regent for his aunt,
Empress Suiko, and he was son to previous
Emperor Yomei, who died around when Shotoku was 13, according to records.
We have no idea who this Emperor is, but he most likely is still someone who ruled in the remaining early chunk of the Asuka period before Shotoku and Suiko's rule, or maybe a late Kofun period Emperor. If we use the 1400-1500 years ago as a hard limit range for how long ago this is, that means he could be as far back as
Emperor Keitai, who ruled from 507 to 531, but he was pretty old at that point, at least in his 50s, from what I'm seeing. The next viable, ignoring his sons that ruled for a few years in their old age as well, is the first proper Asuka period emperor,
Kinmei, who ruled 540-571 and lived for 62 years. He's also noted as 'Often considered the first historically verifiable emperor'... He was 31 when he first became emperor, from what I can tell.
If we want to find an Emperor that was young while he ruled, we'd have to grab the one just before Keitai,
Emperor Buretsu, listed as having ruled from 499-506, and lived for 17 years, meaning he became emperor at 10. This stretches Tsukasa's stated age a bit, though, as this would require her to actually be 1500+, even as of the first publishing date of ToniKawa. (unless we use the fact she says she's been 16 1400 times as the guide, in which case that fudges the range by about 16 years, and maybe lets Buretsu be our man)
All this to say... I honestly have no idea who he's supposed to be, looking as young as he seems to while also being called Emperor, while also having to have ruled at some point between 518 and 593 to retain details we already know about this story.
Anyway... I feel like the Emperor isn't quite the same as Nasa and Shotoku, who may well be connected by reincarnation. I think he's supposed to be yet another soul. He doesn't seem as clever as the other two, but he's certainly smitten, and he shares their straight-forward and earnest ways, I'll say that much.