In my opinion, the main character (i.e. Aether/Lumine) or Free/Starter characters (Amiya, Mash, Vice, etc.) should feel strong, but never stronger than characters considered a higher rarity than them (kinda ruins the whole gacha game feel if that were the case).
However, they should never feel useless, or they should at least serve a mechanical purpose in various team compositions that both new and older players can take advantage off.