" Kakashi weak against Pain?. No, he wasn't, it is just that Pain just had a Rinnegan, the most powerful dojutsu to exist in the Naruto-verse. Besides, Kakashi wasnâ€™t an Uchiha so using his Sharingan a lot fatigued him, making it seem as though he was weak due to how much he had to use his Sharingan in that fight, especially when using Kamui.
By the time it reached the Obito arc, Kakashiâ€™s chakra reserves and experience had grown. Besides, as someone who grew up with the Sharingan eye that he got from Obito, he must have found its weaknesses and used it against Obito, allowing him to fight Obito properly.
Also, you must remember that Obito was not fighting with full strength. Obito was fake fighting Kakashi the entire time so that he can manipulate Kakashi into stabbing him with the chidori so that Madaraâ€™s curse tag is destroyed. So technically, Kakashi would have had a really hard time against Obito if Obito fought with full strength.
If you mean Obitoâ€™s final duel with Kakashi, there are several reasons, one obvious and the others not so obvious, but hereâ€™s my thoughts.
- Pain was fighting seriously against Kakashi, but Obito wasnâ€™t. He threw his own fight.
During the Konoha Invasion arc, Pain was fighting with the intention to destroy Konoha, and Kakashi was intent on stopping him. But when Kakashi fought Obito, the later wasnâ€™t actually seriously fighting him.
The main result of the battle with Kakashi and Obito was that Obito was stabbed through the heart by Raikiri. However, Obito later revealed that Madara placed a seal over his heart in order to more effectively blackmail him. By having his heart pierced, the seal Madara placed over him was destroyed.
All part of his grand plan.
Obito suffered a fatal injury from having his heart pierced by Raikiri. But he was able to subsequently fully heal himself after becoming Juubito, thanks to his new regenerative powers. (Obito outright said to Kakashi: Iâ€™ll let you win the battle, but you will not win the war!)
2. Kakashi was stronger when he fought Obito compared to when he fought Pain.
War arc Kakashi was quite obviously stronger than Pain arc Kakashi, not Kakashi weak against pain. During the war arc, he was seemingly able to use Kamui more frequently. Some speculate he got a â€œzankaiâ€ after coming back from the dead, or he levelled up a lot more before/during the war arc, either through training or experience in the war.
3. Obito (either knowingly or unknowingly) gave himself a disadvantage.
The two of them fought within the Kamui realm. This effectively meant that neither of the two could use Kamui (or had an unspoken agreement not to). While both are skilled with it, Obito had near-unlimited charges of it, while Kakashi didnâ€™t. He was also more skilful and knowledgeable with the Sharingan than Kakashi.
The fact that he instead challenged Kakashi to a basic taijutsu/ninjutsu duel was effectively forfeiting his strengths and instead challenging Kakashi at Kakashiâ€™s strengths. Kakashi is an intelligent combatant and he was about to land clean hits on Obito with just good use of clone jutsus.
(In this instance, Kakashi and Obito clashed using Fireball and Mud Wall. Through the dust, Obito used a thrown kunai to distract Kakashi, and land a hard punch on him. However, another Kakashi bursted out from the ruined wall.
Thinking this was the real Kakashi, Obito turned to him, but it was a Lightning Release Clone, which stunned him, allowing Kakashi to recuperate and land a kick back at Obito.)
Obito also effectively challenged Kakashi for nostalgiaâ€™s sake. Throughout the battle, he was intentionally mimicking their childhood duel. Even resorting to techniques such as the good-old Gokakyu no Jutsu (Great Fireball).
As seen in the flashback, kid Kakashi defeated him in that fight. Obito wanted to reminiscence so he might have unintentionally made a self-fulfilling prophecy for himself to lose to Kakashi.
So conclusion to the above asked question is that Kakashi weak against pain is not right. Source- Quora
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