Radost is from the Bulgarian / Slavic Радост (Radost — joy, happiness). Radost is one of the Bulgarian + Czech + Russian feminine names — central to traditional Slavic Orthodox heritage. Bulgarian feminine name reflecting Slavic Indigenous heritage.
Radost 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 Radost reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.