Mitsuko is from the Japanese 光子 / 満子 (Mitsuko — mitsu light/brilliance + ko child — light child, brilliant child). Mitsuko is one of the Japanese feminine names — central to traditional Japanese heritage. Japanese feminine name reflecting Japanese-European heritage.
Mitsuko 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 Mitsuko reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.