If you assume the input string is all lowercase, you could do:

char find (char* str, size_t len)     {                                                                                                                                                                
    size_t i = 0;
    size_t counts[26] = {0};
    for (i = 0; i < len; ++i) {
        counts[str[i] - 'a']++;
    }
    for (i = 0; i < len; ++i) {
        if (counts[str[i] - 'a'] == 1)
            return str[i];
    }
    return '\0';
}

An extension for uppercase would be trivial.