In a case where it's purely equal like that, the protagonist is usually whoever you sympathize with more. Though it typically just ends up both and thus whoever you're talking about.
So if you're talking about Ryu, Ken is his antagonist. If you're talking about Ken, Ryu is his antagonist.
Though I'd probably say Ryu's the protagonist, anyway, since the story focuses on him more overall.
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SEP Nash, Smelling Like the Vault since 1996
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