i. Whoever plays, is good.
ii. Whoever plays, he is good.
iii. Whoever plays, they are good.
According to my understanding, the 1st is grammatical. 'Whoever' is the subject of the 1st verb as well as the 2nd verb. Even Grammarly says this while I am writing the question. I also found this sentence:
Whoever gets the most votes will become the next president.
But I've seen some people using an extra pronoun (he/they) as I've mentioned.
I think using this extra pronoun is not grammatical.