Krishna's earliest firm attestation is not literary and not Hindu in any form we would recognise, and it is much earlier than most people assume.
The Heliodorus pillar at Besnagar in central India, erected around 110 BCE, is a stone column set up by a Greek ambassador named Heliodorus, son of Dion, from the court of the Indo-Greek king Antialcidas, sent to a local ruler. Its inscription calls it a Garuda pillar of Vasudeva, the god of gods, and describes Heliodorus as a Bhagavata, a devotee. A Greek diplomat had therefore adopted the worship of Vasudeva-Krishna by the second century BCE, and the devotional term was already in use. There is also earlier grammatical evidence: Panini, in perhaps the fourth century BCE, discusses forms derived from Vasudeva.
The literary sources are later and layered. The Mahabharata, in which Krishna is Arjuna's charioteer and the speaker of the Bhagavad Gita, reached something like its present form by the early centuries CE, and the Gita is generally treated as a somewhat later insertion into the epic. The Harivamsa, appended to the epic, and then the Bhagavata Purana of perhaps the ninth or tenth century, supply the childhood: the cowherd village, the butter theft, the flute, the gopis, Radha.
Radha herself is largely absent from the Bhagavata Purana. She becomes central through the twelfth-century Gita Govinda of Jayadeva and the Bengali devotional tradition after it. Much of what is now the emotional core of Krishna devotion is medieval.