Phaethusa (Φαέθουσα) is the Greek name from phaethon ("radiant"). One of the Heliades — sisters of Phaethon — who wept so bitterly when their brother was struck down by Zeus that they were transformed into poplar trees, their tears becoming amber.
Rare but iconic in Ovid's Metamorphoses.
Phaethusa 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 Phaethusa reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.