A billion seems a bit much :D
Rave minor spoilers ahead: When we first really see Sieg he refers to Elie as 3173 because of her tattoo which she read as Elie and he (upside down) as these numbers. They even show some membership card with her name on it upside down so it kind of looks like 3173. I guess it's more obvious if you read it as Latin letters instead of hiragana but it just feels like a pretty stupid idea to hide her name for like... two chapters like this.