Misty is from the Old English mist (fog, mist) + the diminutive -y; popularized by Erroll Garner's 1954 jazz standard Misty. Time magazine's 100 Most Influential People (2015).
Misty 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 Misty reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.