Callista is from the Greek kallistē — most beautiful, the feminine superlative of kalos (beautiful, fine). Callista in Greek mythology — Callisto, the nymph and companion of Artemis who was transformed into a bear by Zeus and then placed in the heavens as the constellation Ursa Major. Callista of Crete — early Christian saint. Callista Clark — American country singer.
Callista 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 Callista reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.