Demelza is a Cornish place name ("Demelza" — a hamlet in St.
Demelza Poldark is the iconic heroine of Graham's series.
Demelza 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 Demelza reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.