Shikai: Absorbs any attack, converts it into energy that the sword can use to cut anything. User's reiatsu can also be used to amplify cutting power if not enough energy was absorbed. When I mean cut anything, I mean ANYTHING.
Bankai: An invisible sphere forms around the user that can basically sense anything inside or near the sphere (5 senses only). Any projectiles (or something with high velocity) gets deflected. It converts the kinetic energy of the high moving object into energy that can be use for the deflection. User controls direction of deflection.
Weakness: If opponent does not attack, then there is no energy to absorb and is just a normal sword. In that case, user has to use martial arts and intelligence to win the fight. The faster the attack, the easier it can be deflected, but if the opponent uses slow attacks, they are still effective i.e. holds, grappling.
Personality: Very peaceful, does not like to fight unless opponent throws an attack.