Thetis (Θέτις) is of pre-Greek origin. The chief of the fifty Nereid sea-nymphs, daughter of Nereus and Doris — forced to marry the mortal Peleus when an oracle predicted her son would be greater than his father (Zeus and Poseidon had both sought her hand and withdrew). Mother of Achilles — she famously dipped her infant in the River Styx, gripping him by the heel, leaving only that one spot vulnerable. The wedding of Peleus and Thetis is the framing event of the entire Trojan Cycle.
Featured throughout Homer's Iliad and Statius's Achilleid.
Thetis 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 Thetis reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.