What is ‘sa iyo’ in English? (in the sense of pabor, kumikiling or sumisimpatiya)

You will use ‘with’

Merriam-Webster’s usage of ‘with’: On the side of, for

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sa kaniya

Kung ayon siya sa mas mababang mga buwis, sa kaniya ako.

If he’s for lower taxes, I’m with him. (Webster’s Encyclopedic Dictionary’s sample)

sa karamihan

Karamihan ay tutol na ang pagkapanalo ni Pacquiao kay Thurman ay marapat sa split decision. Ako’y sa opinyon ng karamihan.

Most people disagree that Pacquiao’s victory over Thurman should be by split decision. I’m with most people’s opinion.