Lleucu is the medieval Welsh name from lleu (light) — feminine of Lleu, the Welsh sun god of The Mabinogion. Lleucu Llwyd was the 14th-c. beloved of poet Llywelyn Goch.
Used as a feminine name in modern Wales.
Lleucu 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 Lleucu reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.