------
## 最终结论
```python
def first_non_repeating_character(s):
    char_count = {}
    
    for char in s:
        char_count[char] = char_count.get(char, 0) + 1
    
    for char in s:
        if char_count[char] == 1:
            return char
            
    return None

# Test cases
assert first_non_repeating_character("abcabc") == None
assert first_non_repeating_character("abc") == "a"
assert first_non_repeating_character("ababc") == "c"
```