While Kizaru does a physical attacks (such as kicking or punching) Does he, or atleast a part of his body transform back into his normal form ? Won't he hurt himself attacking at the speed of light ? I don't quite understand the concept of the speed of light, but I think attacking with the speed of light like that he will break his own body. How is he able to do that?
I think this has something to do with his own physical ability in combination with his devil fruit. But over all I don't think the attacks have a big of a strain on his body to begin with.
His devil fruit the Pika Pika no Mi allows him to turn into light and manipulating light around him. As he can manipulate the light around him to reduce weight he could use this fact in his kick to reduce the weight and make up with speed reducing the actual impact on his own body.
Second of all Kizaru is a highly trained admiral with amazing physical abilities
As a Marine Admiral, Kizaru holds the title of the strongest fighter within the World Government and Marines. He possesses great physical strength, capable of holding down Whitebeard's bisento with just one foot, and is capable of fighting Rayleigh and Whitebeard equally despite their Haki abilities. Kizaru got struck by a "Haki"-wielding Marco, and experienced little damage from the physical blow. His powerscaling places him comparable to Akainu, who managed to deflect blows from Whitebeard with difficulty. source
So he might just be able to sustain it with his training. Which would mean that in longer extending battles his power usage would go down or even not usable.
And my third point would be the fact that it is a Logia type of fruit.
A Logia user can be shattered, split, or otherwise separated into multiple parts, sometimes down to the size of dust, and reform without injury.
In addition to their ability to become their element, Logia users can generate limitless amounts of their element and control it at their whim, giving them fearsome attack abilities. source
The Logia type enhances a user/owner in a totally different scale to begin with. The huge enhancement made by the Logia fruit might already be enough to not sustain injury by his own ability due to the amazing regenerative abilities he has gained with it.
From what I gather, rather than being light speed, he moves at light speed for a short burst of speed. i.e vs Apoo. Light is dodgy, it's both a particle (a photon) and a wave (EMW). I think that the extent of his light speed would be limited to travel speed, and not attack speed. Rayleigh is a little broken as he, like Katakuri, has some form of precognition of ability to read moves very fast.
He would HAVE to transform into a human of part-human form to actually deal damage. Using the Observation Haki, you can narrow it the point here you can point the exact moment he does and prepare a counter.
From Kizaru vs Apoo, we can see that he can only travel in a straight line (unless reflected). Combining both high level observation and Armament Haki, you can pretty much always counter Kizaru.
If you look a the One Piece Film: Z, Z can counter Kizaru because he can predict his action and movement even though he(k) far outspeeds him and used the sea prism to deflect and nullify the attacks.
If you have mass you cannot travel at speed of light, if you have no mass (as photons) you always travel at speed of light. Once part of kizarus body become light, it travels at light speed before it reverts back. As logia his primary ability is to create and control light. When he creates light, he releases energy. Imho, when he reverts back into human form and attacks, he adds that released energy to his kick in form of kinetic energy. Once it connects, his leg would've been destroyed. Instead he instinctively uses second logia ability - turns his leg into light and reforms it.
Bonus, if he pumps enough energy into his leg to travel like 99.999% speed of light, each kick would release energy of circa 100 megaton nuclear bomb, with fireball of 10 kilometers radius and full demolition of 20 kilometers radius
As the wikia states
all of his attacks are based on the element of light; such as firing energy blasts from his fingertips or feet with extreme accuracy and precision. His power is very destructive, causing huge explosions and easily destroying buildings
So basically kicks with great 'energy' .
