The Rasengan uses no hand seals and is based upon expertise with Chakra control. Despite all the answers about its incompleteness and how Sasuke “doesn't need it” and so on, that is all nonsense. He could not copy it because, like Shadow Clone, it's not a technique that can be copied.
The Sharingan if used properly can copy all the signs for ninjutsu or the movements for taijutsu but you still need to be able to use the chakra nature on your own and provide enough chakra or stamina for the technique on your own.
Rasengan is a simple Jutsu yet very hard to achieve. It requires near perfect chakra control which Sasuke has. Sasuke can learn it as well as use it anytime but he has developed his fighting style around Chidori, so there's that.
Kakashi can't copy a jutsu like Rasengan as there are no hand seals. The Sharingan follows movements to the tiny details and by extension allows one to copy their movements from hand signs to basic taijutsu. If the user has the ability and stamina they can replicate the jutsu the signs instigate.
Sasuke's Sharingan was not active to see Vanishing Rasengan, Borushiki's reserves is low and he wasn't using Karma so that Rasengan isn't doing much damage to Sasuke, plus Sasuke is heavily injured and low on chakra so he couldn't dodge it.
No, it was created by Minato. It's very hard to master and incomplete jutsu. A jutsu that is causing significant damage and is able to kill shinobi cannot be called a failure. The attempt of copying jutsu, which within its failure led to the creation of different jutsu, is another story.
Yes, it can absorb Senjutsu chakra. With the Preta Path, Nagato completely absorbed Jiraiya's Sage Art: Ultra-Big Ball Rasengan which was senjutsu-enhanced.
Naruto Uzumaki
Evidently, Naruto managed to accomplish what his father couldn't, and eventually completed the Rasengan. The yellow-haired protagonist is arguably the most extensive user of the Rasengan in Naruto, having managed to create multiple forms of the technique.
Despite not being able to copy jutsus in the Naruto series, he still has access to over 1000 jutsus that he copied when he had the Sharingan. Fans are aware of the fact that Lightning Release is stronger than Earth Release, but Kakashi managed to improvise an Earth Release technique that is unaffected by lightning.
Kakashi is a master of over a 1000 jutsu but these are some of his rarest techniques.
According to Konohamaru the only people that can use the Rasengan were the late 4th Hokage Minato Namikaze, and old man Master Jiraiya. And among the living are only big brother Naruto Uzumaki, and myself.
The Chidori was created by Kakashi Hatake after he failed to apply his lightning-nature to the Rasengan. He intended for the Chidori to be a weapon used only to protect friends and loved ones.
Rasengan was made by a Wind-type, and used by said Wind-type's son, who was also Wind-type. Compared to Chidori, made by a Lightning-type and used by (To my knowledge) a native Fire-type. Sure, you label him Lightning in-game, but most if not all other Uchiha born to the clan have a Fire chakra-type.
Apparently, the TnJ (and the resistance to it) can be passed on to the next generations. This can be seen with Asura descendants Hashirama and Naruto possessing natural TnJ talent while Indra descendants Madara and Sasuke are mostly impervious to TnJ.
One of the most powerful Jutsu in the history of the Uchiha clan, the Izanami is a forbidden Jutsu that can only be performed at the expense of the user losing eyesight in one of their eyes permanently.
Kirin is one of Sasuke's most powerful jutsu, and yet he's only used it twice in the series. The reason is that there is the requirement of existing storm clouds, and the buildup process takes awhile to pull off.
As has already been established, the weakest Hokage of Konohagakure is between Kakashi Hatake and Tsunade Senju. In fact, both Kakashi and Tsunade are quite close to each other in terms of strength in Naruto. However, their fighting style differs a lot from each other.
However, Kakashi was shown to be a ninja prodigy even before he gained the Sharingan, meaning the statement of Kakashi being weaker without the Sharingan is an unjust assumption. Analyzing his current skills and achievements reveals how strong he truly is, even without the visual jutsu.
After losing his Sharingan in Naruto, Kakashi replaced Chidori with its lightning-based counterpart named Purple Lightning, which is nearly identical but without the speed boost and piercing power. Unlike Chidori, Kakashi is able to use Purple Lightning in both close and long-range fights.
1 Sasuke Uchiha
Sasuke Uchiha is the strongest known lightning style user in the Naruto series. Like Kakashi, his go-to Jutsu is the Chidori. However, his arsenal is much more diverse when it comes to Jutsu.
Jiraiya is a ninja from the village of Konohagakure trained by Hiruzen Sarutobi, the Third Hokage. He becomes known as the "Toad Sage" due to his training under the Great Toad Sage in the field of Senjutsu (仙術, lit. "sage techniques"), enabling him to summon toads as allies during battles.
One the other hand, the Rasenshuriken is what becomes to the Rasengan when wind release nature is added to it. It is much stronger and deadlier than the Rasengan and can also take down the strongest of enemies in just one hit. Truly, the Rasenshuriken is menacing.
Rinnegan provides six powers; Deva Path, Asura Path, Human Path, Animal Path, Preth Path, Narka Path, and Outer Path. Among these ocular abilities, Preth Path enables the caster to absorb Chakra. And the Amaterasu being a chakra, Rinnegan can soak it easily.
Anyone with good chakra can be able to use the rasengan. They generally just need have the necessary knowledge, aptitude and willingness to train.
The Rinnegan is able to see chakra and its flow within the body, as well as otherwise invisible barriers, but cannot see through obstructions such as smoke bombs. Possession of the Rinnegan allows one to easily master any jutsu as well as all five basic nature transformations.