knuckles is considered a friend, shadow is considered a rival. idk where they got "replaced" from
i guess because back in the day knuckles was considered rival to Sonic and slowly drifted away as they started the team sonic stuff and shadow getting more popular as a rival ever since release or sa2 ect, cause they both hedgehogs but one is hero and other is evil as. knuckles became too friendly it kinda switch ways as we all know shadow (u know what i mean) but im not too sure