Not all girl have acces to it. Some girls have only the regular counter or red counter and deals only DEF x counter multiplier, even if they do counter and defending. Some other (they often have counter+DEF bonus) can do super counter (or blue counter) when they do counter and defending and deals DEF x counter multiplier x 2.
Girls with super counter are obvioulsy better for counter team. There is only some exception for 6* girls like Saffron who is awesome anyway.
Note that Counter damage are also affected by Weakness and Crit damage. Also, higher defense and DEF bonus increase the chances of defending during battle so, it also increase chance of super counter if the girl have acces to it.
Sorry if I'm not clear, I'm a bit tired.