JS - Return only values matching an ID

I'm retrieving a character list from a database, which requires javascript to work (the rest of the script is in C#) as it uses JSON. I need to basically say only return the values that match the account owner's ID (the character's have that ID stored on them).

This is what I have so far, it returns the entire character list and not ones specific to the account.

 private const string GetCharacterListReduceFunction =
     @"
 function(valueList, keydata, arg) {
     return valueList;
 }
 ";

This is what I'm using to call it

 private IEnumerator GetInfo()
 {
     var bucket = new Bucket(bucketName);
     var getCharacterListRequest = bucket.MapReduce(new JavaScriptMapPhase(GetCharacterListMapFunction),
                                                 new JavaScriptReducePhase(GetCharacterListReduceFunction));
     
     yield return getCharacterListRequest.WaitUntilDone();
     _characterEntries = getCharacterListRequest.GetResult<CharacterEntry>();
 }

The dilemma here is I code in C# and struggled to get even that working. How can I re-write the code to do what I need it to do? I suppose I have no choice but to familiarize myself with JS, but a little help on this would be nice so I can at least finish up what I'm working on.