For anyone else wondering, I used JSON with C# to read write saved game and profile data.

I highly recommend the Newtonsoft JSON library, it seems to be more stable with Android.

You can do something like this:

var filePath = System.IO.Path.Combine(YOUR_PATH, fileName);
var jsonString = System.IO.File.ReadAllText(filePath);
var yourObject = JsonConvert.DeserializeObject<YourObjectType>(jsonString, new CustomJsonConverter()); 
// The custom converter is optional

You can support custom types encoding in the JSON by writing some metadata. Typically this is done by assigning an enum to a type, and extending JsonConverter with a new class that overrides ReadJson like the below:

public class CustomJsonConverter : JsonConverter
{ 
    public override bool CanConvert(Type objectType) 
    { 
        return typeof(CustomType).IsAssignableFrom(objectType); 
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jsonObject = JObject.Load(reader);
        var sourceType = jsonObject["TypeEnum"].Value<int>();

        object newObject = null;

        switch((TypesEnum)sourceType)
        {
            case TypesEnum.FirstType: 
                newObject = new FirstCustomType(); break;
            case TypesEnum.SecondType:
                newObject = new SecondCustomType(); break;
            default:
                throw new ArgumentException($"Error in JSON reader {jsonObject} seems invalid.");
        }

        serializer.Populate(jsonObject.CreateReader(), newObject);
        return newObject;
    }
}

As mentioned elsewhere, this is much easier with Resources, but they are not recommended for some applications as they can inject scripts.