What does Callista mean?
From Greek kallistos/kallistē 'most beautiful', the superlative of kalos, from kallos 'beauty'. Kalliste was a title of the goddess Artemis; the mythological nymph Kallisto bears the same root.
What does Eleni mean?
Modern Greek form of Helen/Helene. The etymology is uncertain: most often linked to Greek 'hele' ('torch') or a root meaning 'shine/bright', with some scholars instead proposing a connection to the moon or a pre-Greek word of unknown meaning. The 'torch/bright light' reading is traditional but not settled.