Both, though the frame of reference is always the Main Character.
The Influence Character's behavior creates greater and greater pressure* for the Main Character to change, which forces the Main Character to EITHER build up greater and greater resistance to the pressure, or slowly have the Main Character's resolve eroded. By the end, the Main Character stays the course, either through conscious choice or perseverance.
MEANWHILE, the Main Character's steadfastness challenges the Influence Character's determination, which either erodes the Influence Character's adherence to its paradigm, or makes the cost of maintaining the Influence Character's paradigm too challenging to hold. By the end, the Influence Character gives in or gives up and changes by adopting the Main Character's perspective (in the context of the inequity).
* NOTE: The pressure increases in part because the Influence Character adapts as the Main Character adopts new approaches to resisting the Influence Character's alternative world view. The changing approaches occur act-by-act, and are visible in the changing frames of reference represented in the four Signposts in the Main Character throughline, the Influence Character throughline, and the MC/IC Relationship throughline.