This is the first time Leo is shown. It's not that it's never been used before, only the first time it's been shown. Leo's original first appearance was supposed to be in the zodiac set but that's been put on hold. However some things have changed that make it a bit more likely to happen in the near future. 

Leo doesn't always look like this. It appears this way because Evey's incredibly angry. But other times Leo has a much softer appearance. You're all likely to see it eventually. 

Leo's a good egg. Hopefully it shows up again some time.