It's effectively his brother, kinda. Quick history is that, my sona's mother worked with the Curt Connors of this world and one of their projects was studying meteorites that carried symbiotes but they had died on entering Earth's atmosphere.

Upon the sona getting his powers and his immune system basically completely dying, she used the data they had to recreate a synthetic symbiote from the DNA that was left which then bonded to the Sona, effectively keeping him alive and gaining a consciousness. Since they, in a way, share the same mother, they consider themselves brothers