The Gathas, the oldest layer of the Avesta and the part ascribed to Zarathustra himself, are difficult, allusive and almost entirely without narrative. They name very few people. Vishtaspa is one of them, and he is named as an ally.
The word kavi elsewhere in those hymns is hostile. It denotes the princes and priests of the order Zarathustra is preaching against, lumped with the karapans as men who obstruct the message and misuse the sacrifice. Vishtaspa is a kavi and is on the other side of that line, which is why his adherence is presented as decisive rather than merely welcome: the class that produced the opposition produced the patron.
Around him the hymns name a small circle. Frashaoshtra and Jamaspa, brothers of the Hvogva family, appear as supporters, and the tradition makes Jamaspa the king's minister and has Zarathustra's daughter married to him, so that the prophet's household and the court are joined.
What that circle represents is the moment a teaching stops being one man's and becomes an establishment's. A religion with a royal patron acquires an economy, a place to perform its rites and a claim on the loyalty of subjects, and Zoroastrian tradition treats the conversion of Vishtaspa as exactly that transition, elaborating it in the later Persian literature into a full court narrative with debates, hostile priests, a poisoned horse and a miracle.
The Gathas themselves contain none of that. They contain a name, an alignment, and the fact that the man is addressed as someone who can be expected to act.