We are made to hear the Witch’s parting words to Kei one last time so we can see his reaction later on: tears of joy. In the last two years, the one thing that has driven Kei is the hope that some day, in a city full of what would be considered miracles in the outside world, he would find the means to bring Souma Sumire back to life.
The Witch essentially confirms that this was the right and proper course all along, because he is destined to meet Souma again, and Souma runs a test to see if he can bring something from a photo world into the real world without that thing vanishing.
The test is a success; a cherry petal from the photo remains even after ten minutes—ironic, considering the nature of sakura. They are something both joyful (because of their stirring beauty) and morose (because they are ephemeral).
But before we get that confirmation the test worked, we’re taken back to the past once more; this time, to when Kei was in the sixth grade and a relative newbie to Sakurada. Even as a little kid he’s a smart cookie, determining before the Bureau rep has to say it that his memory retention ability would work even outside of Sakurada.
Because of this, the Bureau offers him a comfortable life and livelihood in Sakurada…but he can never leave. In effect, Asai Kei is the one ability-user (aside from those who could copy or steal his ability) who could most threaten the city’s very existence by exposing it to the outside world, breaking its Shrödinger’s Cat-like status.
Kei loves this city, so he quickly agrees—discarding his past in the process. Then he experiences a Reset for the first time, and if anything, he’s more excited than ever to be in such a place and such a position. He’s won the lottery, basically.
As Kei sets up the Save Point in the same place where Souma appears in the photo, Haruki resigns herself to the fact Kei will make her and the others forget everything henceforth. As she’s dedicated herself to following his lead, she’s fine with everything, but does wonder if, like Mari, the Souma they save will be the “real” one.
It doesn’t really matter to Kei, but he knows that whoever he revives may not enjoy a “peaceful” life due to her ability. Even so, he must bring her back; follow the script Souma laid out for everyone. He sees the MacGuffin as proof of this: the one who holds it (i.e. Kei, now) will have “all the abilities in Sakurada.”
It was the object whose power was created through rumor and never was anything other than an ordinary stone, and yet it drove Kei to become the protagonist of this story by gathering the ability-users he needed to bring Souma back from the dead.
When the time comes to actually do that, the tension is palpable, and all the tiny hairs on the back of my neck stood up, as the feeling that something momentous was about to take place washed over me. (The show grossly underestimates how hard it is to tear a Polaroid, but no matter.)
Once in the photo, everything happens with a quiet but purposeful haste. Kei doesn’t even run over to Souma, but waits for Sakagami and Murase to do their stuff, then tells Haruki to reset. He’s that sure this will work, even though I wasn’t.
And so it does: by utilizing the abilities of the others, Kei wielded the MacGuffin, rose to become the protagonist of the story, and brought Souma back to life, just as she was sure he would—she shows no surprise in seeing him. Kei hides his delight at having succeeded, in part because he’s a reserved guy and in part because he doesn’t know how the future will progress past this point.
For now, it’s enough that Souma is back. She comments on how he’s grown, both in stature, ability, and in his friendship with Haruki. He also reports how much Haruki has grown, and that he’s come to want her to keep making progress on the road to finding a sense of self, even if it breaks the “pure goodness” he may have fell for earlier.
Souma even set things up with Tomoki so that if she was really Souma and not some kind of copy, she’d receive a voice message meant for her two years in the future. She receives that message, then asks Kei which Souma he’d like to meet, confident he’ll make the correct choice. He is the protagonist, after all. She made him so.