Character Traversal question in C#

CharacterTraversal.cs

        public static char? traverse(string s, bool caseInsensitiveMatching = true)
        {
            var counts = new Dictionary<char, int>();
            for(int characterIndex=0; characterIndex < s.Length; characterIndex++)
            {
                var c_upper = s[characterIndex];
                if(!caseInsensitiveMatching)
                    c_upper = char.ToUpper(s[characterIndex]);

                if (counts.ContainsKey(c_upper))
                    counts[c_upper] = -1;
                else
                    counts[c_upper] = characterIndex;
            }
            foreach (var kvp in counts)
                if (kvp.Value > -1)
                    return s[kvp.Value];
            return null;
        }