String mystr = "";
String original = "aaabb";

char last =  original.charAt(0);
for (int i = 1; i < original.length(); i++) {
  // Some code not relevant
  mystr += last + "" + count; // Here is my doubt.
}

char mystr[1024];
char * str = "abcdef";
char c = str[1];  // will get 'b'
int int_num = 100;
sprintf(mystr, "%s%c%d", str, c, int_num);

"abcdefb100"

strcat(mystr, "xyz"); // now it is "abcdefb100xyz"

char mystr[1024]; // Assuming the maximum string you will need is 1024 including the terminating zero

char lastString[2];
lastString[0] = last; // Set the current character from the for loop
lastString[1] = ''; // Set the null terminator

char countString[32];
itoa (count, countString, 10); // Convert count to decimal ascii string

strcat(mystr, lastString);
strcat(mystr, countString);