There was a few times in the series that really made me think and wonder if Kira and Light was really the same mind.
*Spoilers Ahead*
Let's Start:
I am not saying that Light isn't Kira (didn't use the death note or they was 2 different people like twins) I am trying to say that Light created Kira out of his God complex and to cope with the stress, another personality just was created in the process. Maybe this happened because Ryuk claimed that under no circumstances would he ever help Light out on purpose and not to look at him as an ally. Hell Ryuk even said "I will be the one when it's time to write your name down in my note book" that seriously has to be stressful knowing that especially since Ryuk said he was doing this because of "Boredom" meaning if Light slacked off, Ryuk might kill him for that too even if he wanted to stop..he simply couldn't just stop anymore.
Light had shown from the very start of Death Note, that he had ideals, a certain way of thinking before he even picked up the note book, so there is no questioning that Light was the one at least in the start that was in charge of his own mind.
Kira's True First Apperance:
In episode 2 was where Kira really began to take form (before being named as Kira or recoginized) and was even at his strongest in Lights mind. Notice how Lights decsion to kill the fake L here was also so weird and reckless. I guess we could argue his moment of weakness was his biggest mistake and he was much more careful later on in the series because of this one particular mistake... but it still seemed just so maniac how he acted and the things he said at that moment.
When To Notice Kira:
Throughout the series there is a alot of-
Where Light is in deep thought or planning something his hair turns red, the symbolism of this may mean more than it seems. I think that this is the crucial times that Kira comes out to play., epescially when his eyes are red too.
The Deleted Scene:
There was also a deleted scene that got cut from the anime, it was of L's funeral.
I think if you see that scene where he said "Dad I promise I am going to find Kira and make sure he is executed"and how honest he sounded with that and then right after that when everyone walked off & left and you see this 
Did Light Fake His Dads Death Reaction?
Let me remind you all of this moment:
You can see Light here crying like a baby, sure it could all have been an act but he really gave off the vibe it was genuin here.
Maybe Light overpowered Kira just enough to let himself feel emotion and empathy for his dying father and true Light Yagami
is seen for a few moments.
When Light Got Shot:
Light after being shot by Matsuda, Acted as if his father was a joke, even spoke about his dad like he didn't even properly know him, going as far to call him "Souichiro Yagami" just right after calling him "My Dad.." "My dad..oh you mean Souichiro Yagami right Matsuda?" almost like he had to correct himself quickly. Which when you think about it, is rather weird. Was it Kira talking in that moment instead of Light. Considering just earlier he cried when his father died... Light has also never once talked bad about his dad in the series till that moment.
He also said after Matsuda shot him to the ground.. "Where are you Misa?" "Where's Takada..." "Somebody..what do I do now.."
Like come on.. Light made Misa give up her memories and Light had Takada die in the back of that Truck in order to kill Mello.
How the fuck did Light not remember this?!? well I will tell you why..because Light didn't remember it because Light didn't do it.. it must have been Kira! his split personality, because they must not share the same memories for certain things that happen. I don't ever see Light forgetting something that important but then again..maybe he was just in panic when he said those things, most people would cling to any kind of hope to get out of that situation I imagine, but still.. very odd stuff to say in my opinion, mainly the bit he says"Where's takada".
Did Light Have Memory Loss Or Personality Loss?
Light had to battle the entire time to pretend he isn't Kira, so maybe he just overtime literally forgot who he was. Was he Kira or was he Light? was he both? Yet he simply couldn't be both in his mind, as he had to at all times make sure he had completely seperate them and live 2 lives so he woudn't be captured.. this is where I think Kira truly did become its own personality within Light. When Kira left Light after giving up his memories, Light said a few times later on that Kira reminded him of stuff he thought of and his ideals, but he proved to be more like L than Kira during this time period of memory (or personality lost) so does that mean Kira was a true split personality after all? and isn't something that Light would actually be in different circumstances.
Kira when returned back to Light (or his memories lol) even made a joke about saying "How predictable I really am huh..even wearing this watch"
Kira Died, Before Light Died:
Episode 37, the final episode, is where we see Kira completely show himself and then vanish completely as Light slowly fades away as he dies too. You can tell this because Light looks back at himself, his past-self (or true self) and he doesn't see Kira within himself at that time, looking at what things could have been like before ever picking up the death note. It even shocked him and made him jump a little bit as his past-self walks by. I personally do think that Light did do many horrible things himself, but Kira was the main one behind it, with Ryuk the puppet master of Light without him ever really noticing it. Anyone else thinking that Kira left Light the moment he was shot by Matsuda?
If Light Had Survived?
Sadly we will never really know because Light never survived, maybe if he had of survived he would have been a completely different minded person again, even leading up to his prison sentence or execution. Light even saw L in his mind right at the end before Ryuk killed him. I personally think this was either proof of the afterlife or Light regreting killing his one and only friend he had. That or L was like "haha fuck you bitch, won't be seeing you later in nothingness buddy" Lol but I doubt that and that still proves of an afterlife (even though the manga says otherwise)
Anyway, thanks for reading my crazy theory which is most likely not true, can be proven and was pointless. However since watching death note 14 times, why the hell not have some fun with the content and get deep with it. Maybe someone else can add to this theory, or maybe it will completely change the way they felt about death note, light yagami, kira and the ending all together.
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