Sync Pairs gain stats whenever they level up, with stats growing linearly between two level breakpoints at a time. At levels 1, 30, 45, and 100, stat growth changes to a new, still linear value until the next breakpoint is reached.

(Note that stat growths per level can decrease at a new breakpoint)

For example, here is the HP stat growth for Rosa and Snivy from level 1 to level 100.

Red bullets denote breakpoints at levels 1, 30, 45, and 100 respectively.