For some, Mirai's disinterest in life and uncertainty about her future at the beginning of the show is a relatable feeling.
At the end of episode one when the earthquake strikes. Mirai clings to a railing and is helpless to do anything but call out Yuuki's name, because he could be dead and she could never know because she let him go alone.
At the beginning of episode two, Mirai begs for help and asks the people leaving the shopping mall if they've seen her brother. They all walk past her, having their own problems to deal with.
Mirai recklessly jumping down a wrecked staircase with no regard for her own safety when Mari suggests that they look through the stores downstairs.
Combined with Squick and Nightmare Fuel, at some point, a dog is seen in the background carrying a disembodied hand in its mouth.
Mirai snapping at Yuuki and throwing her phone, causing Yuuki to run off.
The two crying out loud together in a parking lot when Mirai apologizes to him. Doubles as Heartwarming and Funny in a cute way, since it attracts the attention of the old folks that live nearby.
In episode five, we see that one of Mirai's classmates has lost her mother in the earthquake. She only appeared for a brief scene and Mirai herself said they weren't even friends however it's still tragic, especially since it was so close to home for Mirai and it serve as a Mood Whiplash after one of the few times we see her genuinely happy.
In the same episode we learn about the couple whose grandkids died during the quake.
The grandmother is denial and believes that Mirai & Yuuki are her grandchildren, so she keeps trying to approach them. Her husband has to gently urge her to go back to their camp.
At the end of the Breather Episode, when Yuuki faints. Mirai's reaction and Mari running to get medical help really seal the deal.
In episode nine when Mari believes that her daughter and mother are dead. The reunion is heartwarming and sobworthy.
The end of the tenth episode, ("Listen, Sis") when it is revealed to Mirai that Yuuki has been dead for the last two episodes.
The flashbacks are silent, but no words are necessary when you see Mari desperately talking to a doctor and breaking down on her knees as we are provided a Wham Shot of Yuuki's body getting black-tagged.
The final episode, ("Dear Yuuki") this in spades.
After finally reaching her house, Mirai searched for her mother, and found Yuuki sitting on his bunk bed. Worse, as he spoke, his voice slowly disappears, and so is he, leaving her crying and begging for him not to go, as shown on the picture above.
Mirai: I can't hear you. I can't hear you, Yuuki. Don't go. Don't leave me...
When Mirai is finally reunited with her mother and they both embrace each other and sob, both of them in relief for at last seeing each other again and Mirai also in heartbreak as she finally comes out of denial about her little brother's death, as she saw him disappear on the doorway as they hugged.
Mirai and her parents crying for Yuuki in the hospital. Enough said.
Mari returning Mirai's cellphone, which revealed Yuuki's final message to his sister. In particular, the scene in which they both hug in Mirai's bedroom.
Mirai: What do I do? Yuuki's gone now. I miss him so much!
The flashback scene of Yuuki's life before the Earthquake. The music and "thank you for being born as my little brother" ensure it's impossible to watch without waterworks flowing.
The vase of flowers on Yuuki's desk, having seen his classroom in the previous episode.
The credits are both this and Heartwarming. We see minor characters in their lives after the earthquake, including the couple that lost their grandkids in the earthquake sitting together and watching the ocean.
