Riot Games usually keep their upcoming champions well hidden but a partner program has accidentally leaked the new support whose name is Yuumi. She will be an enchanter support champion and there are already a few in-game pictures.
Yuumi has not been officially announced yet but the leaked images show the creature to be an adorable cat that is apparently riding a spell book. The leak only provided the images and no actual gameplay or ability descriptions but there are a few other hints Riot Games dropped prior to the leak.
The first time around Riot spoke about Yuumi, they did not name her but they did state she is whimsical and is more in line of the traditional enchanter supports like Janna or Nami.
They stated that the nature of this champion will be a complete opposite of Pyke who is dark, brooding and hungry for blood. A whimsical cute cat definitely fits the bill here.
Therefore, the where Riot talked about Yuumi inadvertently confirmed the legitimacy of the leak that came when Nasteey accidentally posted a thumbnail for a guide he is preparing for the champion. Given that the YouTuber recently , it is highly likely this is when he got the footage of the new support.
Furthermore, this wouldn't be the first time that League of Legends Partnership Program leaked upcoming content, since Professor Akali also leaked Akali's rework before it was officially revealed, which is another hint towards the Yuumi leak legitimacy.