Siwan is the Welsh form of Joan — from the Hebrew Yochanan (God is gracious). Siwan Phillips — modern Welsh figure. Siwan Davies — Welsh academic. Princess Siwan — Welsh royal heritage naming.
Siwan 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 Siwan reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.