A quick trivia post today! Some of you may have read a recent Life of Heroes chapter that revolved around this too, so I wished to note it for posterity.
This post only covers the spellings in JP and not the implications/reasons for the name, so is mostly spoiler free.
While both the male and female player character in Three Houses is named “Byleth” by default, in Japanese, they actually have two different spellings:
Female Byleth = ベレス (lit: Beresu)
Male Byleth = ベレト (lit: Bereto).
Note the first two characters (bere) is the same, with the su and to as the differences. One sounds more feminine, and the other more masculine too.
Of the two, male Byleth’s spelling of ベレト (transliterated as “Beleth” in this case) is the one often used to refer to the Byleth (in demonology) that the English spelling is referring to, as evident by the JP wikipedia entry.
The female spelling of ベレス yields primarily Three Houses related results. In some cursory searches, it does not seem to be a reading for the being above (which states ビレト bireto for Bilet and ビュレト byureto for Bileth as alternate spellings). So there is a differentiation in Japanese, whether it was to give a masculine/feminine inflection in the name is up for guessing though.
Both male and female names can still become “Byleth” (or Beleth) however. The su in beresu (female name) can either come as “s” or “th”. Fire Emblem fans should be familiar with this double reading already: think of Marth’s own name! マルス (marusu) which can become Mars (like the planet/god of war) or Marth (like the Hero-King as we know him). It’s actually why Marth ended up being named “Mars” in the anime. It was not wrong, per se, but an alternate reading.
Beyond Fire Emblem, a famous example is Final Fantasy 7 and the disagreement whether the character should be referred to as Aeris or Aerith. Basically, transliterating that su at the end of the name can be a little tricky!
Back to the topic at hand, it should be noted that the English spelling of “Byleth” is just one of many spellings to refer to said reference (Beleth, Bilet, Bileth, and Bilith being some others). “Beleth” would fit the way the male name in JP transliterates too, so they are, in the end, referring to the same thing, especially keeping in mind the su and how it can be “th” for the female name to match “Beleth,” and thus the male name too.
So the curiosity is why localization may have went with a single spelling “Byleth” for both male and female characters, rather than opting to use some alternate spellings to achieve the same effect as Japanese. Male could be Byleth and female Beleth, for example. They are the same reference, but that slight difference in spelling may help one sound more masculine and the other more feminine, or at the very least, reflect that difference in Japanese.
One theory is that perhaps localization wanted to make it gender neutral, and so presented just one spelling for both. Another possibility is to make it consistent with previous “avatar characters” in Fire Emblem games, such as Mark, Kris, Robin, or Corrin. The Japanese only deviated from giving them the same name this time around.
On a final note, it’d be interesting to see the mindset of why “Byleth” (this particular spelling) was chosen too, considering the selection available!
It’s unlikely we’d ever get an answer, but hey, now you can understand the original joke behind Ch 95 of Life of Heroes!