void square(int side, char fillCharacter);
printf("Enter the square's side:");
printf("Enter a character: ");
I've never been able to store scanf's results in an int, or char (maybe my own lack of experience). however, you can define a temporary char pointer
pass that to scanf:
scanf("%s", s); // get squares side
scanf("%s", c); // get character
and then use atoi to convert that to an int.
Then, only pass the first element of the string to your square function.
Also, you'll need to include stdlib.h.
This worked for me, but not being too big into C, there's probably better, or more correct ways to do it. Unless I changed something and forgot, your square function is, well...functional