Sayu is from the Japanese sa (沙 — sand) + yu (祐 — protection, blessing). Sayu Yagami is a character in Tsugumi Ohba and Takeshi Obata's Death Note manga (2003-2006) and 2006-2007 Madhouse anime adaptation. The Death Note manga has sold over 30 million copies worldwide; the anime has been broadcast in 70+ countries. Her psychological breakdown following her release (her trauma rendering her catatonic for an extended period) remains among the most-discussed character arcs in modern psychological thriller anime.
Sayu 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.
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In Pythagorean numerology the letters of Sayu reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.