Invincible Evolution: Starting as a Chick

Kieran opens his eyes in a world where humans bind beasts by contract, and the body he has landed in is a chick rated F, the lowest grade there is. Serena, a beastmaster rated S, takes one look at him and throws him away. What she does not know is that the chick carries an ancient phoenix bloodline, and that waking it also wakes a system that lets him grow by devouring what he kills. He finds Aria in much the same position he is: rated at the very top for potential, and abandoned by the beast she had contracted. They sign with each other. From there the chick begins changing shape, first into a red kite trailing embers, then into a sun-bird that burns as it flies, and further still. Their contract runs both ways, so every step he climbs pulls Aria up with him from her E rank. The people who laughed at a discarded chick keep running into him. Demon beasts come at them in numbers, and behind the beast tide is the lord of the Abyss, who has plans of his own for the region.