This video gives you a clear and brief answer about How did Kakashi get the Mangekyou Sharingan?
Hatake Kakashi doesn’t belong to the Uchiha family and has no relation with them but he got the Sharingan and Mangekyou Sharingan, when he first revealed his Sharingan many fans have been shocked and couldn’t understand why and how? So what happened? and how did Obito get Mangekyou Sharingan? What are Kakashi Mangekyou Sharingan’s abilities?
If you watched the anime (all of it) you’ll know that those questions have been answered by Kishimoto sensei so I’m not bringing anything from my head, I’m telling you what Kishimoto sensei said. So let’s get started.
First How How did Kakashi get his Sharingan?
Kakashi as a young person was a member of the Minato Team as a Shinobi, his sensei was Minato Namikaze and his teammates are Obito Uchiha and Rin Nohara. Obito as an Uchiha was very weak but he love himself and he always thinks that he is a strong man and just by awakening the Sharingan he will be undefeatable.
Kakashi was stronger than him, he was protecting him until he got injured in his eye then Obito awakened his Sharingan, However, he didn’t last forever because after some time when he was about to die he suggested to Kakashi that he must take his Sharingan. This is exactly what happened and Kakashi got his Sharingan.
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How did Kakashi get Mangekyou Sharingan?
In brief, Kakashi and Obito both awakened Mangekyou Sharingan when Kakashi killed Rin Nohara by mistake using his Chidori.
Rin wanted to die because of what happened to her and because she is holding a monster inside her “Isobu“. A Three-Tailed beast by the order of Madara during the Third Great Ninja War.
This mist ninja who did it to Rin applied a Forbidden Individual Curse Tag that cannot be removed until Rin and Kakashi return to Konoha so the monster can come out.
In your opinion why did Madara choose Rin exactly and not someone else? it’s because he wanted Obito to see this hard show which is Rin dying so he can become stronger besides hating Konoha and trying to revenge. Do you think he succeeded? yes, he did.