Hyein is from the Korean hye (혜 — wisdom, intelligence) + in (인 — benevolence, humanity). Hyein in Korean tradition — appears in numerous Korean feminine compound names. Princess Hyein — modern Korean royal heritage naming.
Hyein does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Hyein reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.