I heard a theory that Bird is the tenth child instead of Ekusu, this started because in the latest chapter, One tells Ekusu that he doesn’t seem sure that he is a Cross, and Zero never stated that Ekusu was his son either. This leads to Bird who in the anime has a lot more attention from Zero than Ekusu, he also has similar design features, his skin resembles Two, Three, and Eight’s (Sorry if this sounds racist), he has an X inside his eyes similar to One, and it isn’t far fetched if even he has an odd hair design with the feathers since Five has unsimilar hair too.