A few minor tweaks that might help Yuumi be less toxic
W: You and Me!
New: If Yuumi's Anchor takes damage equal to 25% of their max health in less than 2 seconds, Yuumi is kicked out and stunned for 1 second.
E: Zoomies
Active: Yuumi Heal power heals herself, with the base amount increased by 0% − 130% -> 0% -- 90% (based on target's missing health) and the scaling amount increased by 0% − 300% -> 0% -- 200% (based on target's missing health), and gains 25% (+ 10% per 100 AP) Movement speed icon bonus movement speed decaying over 2 seconds.
Okay, I'm not positive about this one, but I think it'd be an excellent step in the right direction.
(There, I changed the title. Happy now, community?)