Mitsuki is from the Japanese mi (美 — beauty) + tsuki (月 — moon) — "beautiful moon" or alternatively mitsu (光 — light) + ki (希 — hope). Mitsuki Endo — Japanese figure skater. Princess Mitsuki — modern Japanese heritage naming.
Mitsuki 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.
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In Pythagorean numerology the letters of Mitsuki reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.