Ransom Stoddard

The impact character could be the antagonist.

For example, in The Man Who Shot Liberty Valance, Tom Doniphon's hard-earned realism forces Ransom Stoddard to confront the limits of his idealistic beliefs about law and civilization.

The impact character could be the mentor.

For example, in Mistborn by Brandon Sanderson, Kelsier challenges Vin's deeply ingrained distrust by showing her a radically different way to understand loyalty, belonging, and her own worth.