Kurama

kurama (the nine tailes) is the last in the tailed beast group, and said to be the strongest. he is a big part in the story line due to the akatsuki wanting him and kurama constantly taking control over naruto, but eventualy, sasuke takes control over him after yahiko sealed kurama inside him, and his sharingan allows sasuke to control him better than naruto.

background
during the dawn of the golden shinobi world, the six path sage divided the 10 tailed beast's chakra into nine seperate beasts and sealed the ten tails body in the moon. the nine tails was one of those beasts, his chakra is one of the highest, and as the tailed beast were skattered across the world, the nine tails kept its distance from humans, but madara realised that the mangekyou sharingan could control kurama, so he used him to attack hashirama senju, but when he died, kurama was let loose back into the world as a free spirit, later on, kurama was sealed inside mito uzumaki and then kushina uzumaki, but tobi removed kurama from kushina and gained control over kurama and made him attack the hidden leaf village, then minato namikaze sealed him inside his son, naruto.

healing
kurama has the ability to heal himself, and due to his connection with his jinchuriki, he can heal them to, the smaller the injury, the faster the repair.

tailed beast ball
even though he has not got all of his chakra, kurama can create a huge tailed beast ball which is much larger than himself, the power of it is the same as an ordinary tailed beast with all thier chakra, his chakra levels are enormous, and that can make his tailed beast ball increadible powerful.

chakra mode
when naruto first beat kurama and placed his chakra inside himself, kurama was locked in a new seal created by naruto, now naruto can use the nine tails chakra mode which covers his whole body in chakra and forms symbols which resemble the six path sage's necklace with the 6 tome's and an enhanced seal. kurama can't stop naruto from entering this mode as the chakra naruto takes is not from kurama.

later on, naruto and kurama make a truce and naruto unseals kurama, the deal was that kurama lets naruto use all of his power when he needs it, this made the chakra mode more deadly and powerful, it has taken a slightly new form, he now has a chakra coat with a black interior. this mode also has the ability to summon kurama in a chakra form, he also has symbols on him in this form, but naruto has almost complete control over him in this form.

sasuke's chakra mode
like naruto, sasuke managed to take control over kurama, but sasuke had more control due to his eternal mangekyou sharingan, and his chakra mode is slightly different, he has a skull-like mask/helmet, a spike-spine on the back and can change his hands into swords. his form appears to be stronger than naruto's, this could be due to sasuke having more control over him than naruto.