Petronilla is the Latin diminutive feminine of Petronius — meaning "little rock," from the same root as Peter. Saint Petronilla was a 1st-century Roman martyr, traditionally identified as Saint Peter's daughter.
Petronilla is widely used in Italy.
Petronilla 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 Petronilla reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.