Yuumi, The Magical Cat, was introduced recently with Patch 9.10. As with all new champions, she received considerable play throughout the elos in the hopes that she would be powerful. However, she has underwhelmed so far, sporting the lowest win rate in the game so far at around 35%, 10% lower than Ryze and Kalista (who both nonetheless hold considerable power in competitive play), as per Lolalytics. Only release Syndra sported a lower win rate at 27% due to her buggy nature, which once cleaned up, was boosted considerably.
In the case of Yuumi, her predicament appears to be the result of low numbers in her kit overall and her inflexibility. Riot have implemented the former strategy multiple times; rather than risk an overpowered champion on release warping ranked play, an underpowered champ can be released that will have less play due to their non-viable nature and thus not distort the ranked ladder. Buffs would then be implemented over time as was seen with Sylas and Kai’Sa. Riot have already applied a hotfix to Yuumi, a day after her release, buffing her mana regen to combat her high mana costs and various numbers in her kit across the board to grant her greater agency. Time will tell whether these prove sufficient or if further buffs will be employed, though the latter proves more likely based upon previous patch histories on new champions.
Unlike previous new champions, Yuumi is viable in one role and one role only: support. Whilst Sylas, Neeko and Pyke did experience issues on their release, new builds (Tank Sylas, On-hit Neeko, Tank Pyke) and their flexibility enabled them to become viable at all levels. However, Yuumi is designed as an enchanter support, with her Attaching mechanic restricting her to the support role. With this new mechanic, Yuumi’s playstyle is much more different than anything seen before; many Yuumis attach themselves onto their ADC and remain attached to the point of offering no lane pressure through combining autos and abilities as is the case with other enchanters. In theory, Yuumi should alternate between attaching and detaching to offer more standard play through the laning phase. Whilst Yuumi does hold considerable power in the late game with lockdown and the ability to buff a hypercarry, she and her bot lane partner struggle to get to that stage.
Overall, Yuumi does offer some unique and original innovations into League play, but her numbers are simply far too little for her to be considered viable. Besides Yuumi players getting used to playing as her, bot laners also have to get used to playing with her and the possibility of effectively 1v2ing. Over time, eventual buffs and a clearer picture on Yuumi macro and micro should propel her into something resembling viability (read: not a 35% win rate).