Riot Games have revealed Yuumi's abilities, not long after the upcoming League of Legends' support champion was leaked. This tiny ball of fur will act like a proper cat - it will attach to other beings and play with them, providing buffs.
Riot Games have offered both a visual representation of Yuumi's abilities on the teaser trailer above as well as a somewhat more accurate description of each ability on the official website. No exact numbers for base damage, healing, shielding or any of the scaling have been provided though.
Yuumi has a magical book that follows her everywhere and transports her, should the need arise. The cat can use other teammates for transport too though, since her W ability, You and Me!, attaches Yuumi to an allied champion. She starts the game with one point in this ability so it's possible to influence other abilities through it.
When Yuumi is attached to an ally, she cannot be targeted by anything except for the turrets and all her abilities affect the allied champion instead. Skill shot projectiles are launched from that champion's position.
The passive ability, Bop 'n' Block will periodically empower Yuumi's basic attack to restore some mana and shield her. If she's attached, the ally instead. This shield will persist until it's broken - it does not have a set expiry time.
Prowling Projectile, the Q ability, will fire a missile that will slow enemies and deal bonus damage if it travels long enough. It will be fired from an ally if Yuumi is attached and she can dictate the projectile's movement in this case.
E ability is Zoomies, which has two charges. Yuumi heals herself and gets a burst of movement speed but the spell will target the ally she's attached to, if she's not on her own.
Yuumi's ultimate is Final Chapter which will cause the book to keep firing a skillshot similar to Sona's Crescendo. It will keep dealing damage and will root enemies that are hit several times by the ability. During this period, Yuumi can freely move, cast spells and attach herself.