Yuumi takes part in League of Legends from a position no other champion can hold: riding on an ally. You and Me! launches her onto a teammate and renders her untargetable by everything except turrets, so she spends most fights as a passenger, placing spells and keeping health bars topped up while her host handles the movement.
Feline Friendship forms a connection with whichever ally she rides most often, and that Best Friend gets enhanced versions of her kit: You and Me! gives them on-hit healing and raises her heal and shield strength, Prowling Projectile always lands its empowered slow, and Final Chapter gains added guidance. Zoomies produces a shield and grants move speed and attack speed, which transfer to the ally whenever she is attached, giving a tidy tempo tool for both chasing and disengaging.
Piloting from the passenger seat
While attached, her main skill test is Prowling Projectile: a steerable missile that deals bonus damage and a stronger slow when it flies for at least 1.35 seconds before hitting, so curved, patient throws are rewarded. Final Chapter sends out five damaging, healing waves while she is still able to move, and she can re-attach and cast Zoomies, which helps stretch out teamfights.
Rated difficulty 2, Yuumi is mechanically forgiving but strategically demanding. Her value depends on when she attaches and which carry she chooses to empower. She suits players who would rather enable a hyper-carry than play one, usually as a support and now and then flexed to mid. The downside is that while attached she provides no body on the field, so her team effectively fights a player short until the healing starts paying off.