Prahlāda is the boy from the Holi story, born among the Daityas, who never stopped believing. Kṛṣṇa names him: of the Daityas, I am Prahlāda.
Going deeper
10.30 names four things Kṛṣṇa is: of the Daityas, Prahlāda; of reckoners, Death; of beasts, the lion; and of birds, Garuḍa, the son of Vinatā. The line comes in the middle of the list in chapter 10, after 10.29 has named Ananta, Varuṇa, Aryamā and Yama, and before 10.31 moves on to the wind, Rāma, Makara and the Ganga. The story presents Prahlāda as the boy of the Holi tale, born among the Daityas, who never stopped believing. The verse itself says only that among the Daityas, he is Prahlāda: of all that race, this is the one it names. The list works the same way line after line, holding up one name as the finest of its kind. See chapter 10 and what the Gītā says about devotion.