Who says the spotlight always has to be on the main character? This arc clearly isn't just about Mantaro, for he's not even the central focus here. Besides, Mantaro had plenty (maybe too much) of time to shine in Nisei. I'd say it's perfectly justified for Kevin to finally play a role in this arc, considering how long he's been absent.
i think everyone has already taken Mantarous place as Main Hero i mean lets face it who beat whom in the finals of the Olympics? Kevin proved to the world hes stronger than Mantarou besides i never liked Mantarou from day one especially after his bout with Mars