Kei goes into the arc expecting to overturn canon. She fails. Her personal Butterfly of Doom kicks in, and Kakashi is grabbed instead of Obito and despite Team Minato's best efforts. He's put through a field interrogation that results in the loss of his eye. While Kei and Obito do manage to kill the two Iwa jonin, because they underestimate the two chunin, Obito is still caught in a rockfall while saving the lives of his teammates. He has the longest death scene of any character in the story.
Obito: "Kei...go. Please...don't let him die. He needs to...to keep his promise." Kei: "N-No, I can't..." Obito: "...I'm not going...anywhere, Kei..." Kei: "...I'm so sorry. I'm sorry I couldn't... I should have done something! You shouldn't be stuck here..." Obito: "Heh... That's just l-like you, Kei... G-Go. Move forward..." Kei: "I...okay. Okay. You win." ... Kei: "I know... (I’m sorry.) Sleep well, Obito... I...we'll see each other again soon." Obito: “You...know it...”
But this is impact is eventually mitigated by Obito's reappearance one timeskip later.
The Commencement Arc:
The body count for the arc includes Kei and Hayate's mother Miyako.
In the aftermath, Kei and Hayate spend the next few chapters (and a timeskip) grieving in various degrees of visibility.
The Bloodlines Arc:
Obito finally has a long-overdue breakdown due to the group finding out that Madara died not long after the events of the Commencement Arc. The latter character's death isn't viewed as a tragedy by anyone else, but they stay to comfort Obito as the Stockholm Syndrome finally rears its ugly head.
The entire lead-up to the arc gives the readers a good view into Kei's psyche, where her self-worth is tied up in her ability to predict the "future" and takes a nosedive once all the changes settle in.
During the battle for Sorayama, Kei's team—consisting of Gai, Genma, and Ebisu—ends up getting split to help the civilians evacuate while dealing with threats. While Gai and Ebisu are fine, Genma is caught unawares by Orochimaru and gets beaten half to death. Not long after he's saved from immediate danger, Raido's team runs across the field hospital. His reaction isn't optimistic.
I lean down over Genma’s face and press my lips to his forehead. If I concentrate, I can just barely feel his breath, weak though it is, brush against my right ear. He’s alive, still. But that’s all. I just wish my thoughts could get through to him like this, but they can’t. I’m sorry, Genma. Even if you wake up I...don’t see how this will work out. Goodbye.
During the last chapter of the Memory Arc, Kakashi confronts a berserk Kei who has completely lost control of Isobu's chakra as the seal breaks down. He does his best to talk her down, but it fails. And when he doesmanage to stabilize the seal for a moment, she's almost immediately stabbed in the chest by Kusanagi right in front of him. Poor kid.
Ocean Stars Falling
The lives of the jinchuriki are generally unhappy affairs, brought to the forefront by the number of them in the cast who can finally commiserate over their experiences.
Though Yugito is very loyal to her village, she's as affected (read: guilt-ridden) by the offhand mention of being used as a human weapon as Kei is.
Side stories reveal that Utakata's childhood was very isolated, and he values his pirate friends after their deaths because they accepted him and Saiken instantly.
Han outright says that he'd much rather continue babysitting a tiny civilian milkmaid on an island in the middle of nowhere than go back to his village.
Gaara is clearly much happier with the Straw Hat Pirates than he ever was in Sunagakure, and even admits that he and his siblings aren't close.
Kei's childhood was sacrificed for the sake of keeping Naruto and Kushina safe, even if she made the choice willingly.
Ace's POV chapter takes place in Impel Down as he's awaiting execution. While chained to the wall with sea prism stone to keep him from escaping, he has a lot of time to think about how much he's failed his loved ones by being captured. He mentally apologizes to the Whitebeard Pirates, his brother, and even his new jinchuriki friends for leading them into trouble. The only thing that lifts his mood is when Jinbei is crammed into his cell, and Ace ends up telling the most reasonable of the Seven Warlords about Luffy.
Shell Game
More than once, Kei makes a Call-Forward to what is referred to as the "hell year" by the readers. The issue comes up each time Kei makes a remark, mental or otherwise, that downplays her life's value or her past traumas.
Hitoshi's self-esteem is clearly through the floorboards, thanks to constant rejection of his Quirk and therefore him as "villainous." He tends to lash out verbally at people nearby, and was well on his way to not having many friends before Kei sorts him out, even if she's not necessarily the most forthcoming person.
Kei and Hitoshi have a falling-out late in the Sports Festival, and Hitoshi briefly has a moment of pure emotional confusion when, instead of cutting ties forever, Kei leaves a token of their friendship with him while he's unconscious. Until Homura shouts him into order, he doesn't even know how to begin approaching patching up any hurt. He clearly hasn't had many friends before.
During the raid on the training camp, the UA students' terror amid the attackers really shows that they are just kids learning to be heroes. They panic, cry, or otherwise freak out while in life-threatening peril, even while fighting back. For Kakashi's part, this means he defends the students to the point of risking injury that lays him out flat, but Kei instead wishes that she had some way to comfort them—but as Turtle, she can't.
Kei and Hitoshi have a difficult conversation while visiting a cat cafe, because Kei is not going to be in Tokyo forever. They've grown close enough that the thought of Kei leaving permanently hits Hitoshi hard. He's clearly not dealing well with it a chapter later, either.
Dig Two Graves
Lotus Pier falls in the very first chapter. For those in the know, that's the moment Jiang Cheng, Jiang Yanli, and Wei Wuxian lose everything and everyone they know. It's mitigated a little by the shinobi organizing to hide those cultivators who were in the field, but they spend months convinced that they're all that's left.
Tomoe's personal history is similarly fraught, which is one reason she doesn't object too much to joining the Sunshot Campaign. As explained in The B-Plot, her entire clan was massacred when she was around the same age as the Jiang siblings, with the only other survivor being her toddler nephew. Her surviving family (her married-out sister) wants nothing more to do with her. Her only overarching goal is to track down and kill the people responsible, no matter where that journey takes her. The only consolation is that Shinta followed her across the sea.
Shinta disappears in Qishan after the cultivator carrying him is shot out of the sky. Because of the sheer number of fierce corpses in the area, the Jiang and Jin cultivators who survive the battle have to flee without retrieving any of their comrades' bodies.
Minato and Kushina arrive in the story—not as fully-fledged shinobi, but as scared genin being shuffled from place to place by Deep Cover Agent adults who are desperate to keep them alive.
Wataru has no actual idea how to deal with children because of his past experiences, and has a difficult time coming to terms with being their primary guardian. He's afraid of hurting them by accident, constantly stopping himself from wondering what trauma they're already carrying and trying not to pry.
Wataru gets his first-ever letter from the Sunshot Campaign's leadership.
Chen Hao of Lanling,
I regret to inform you that Wu Tao disappeared one month ago in Qishan, during a mission with the Jiang and Jin forces to destroy a Wen outpost. During the retreat, he fell off a sword and his body was not recovered. Upon learning this, Wu Xue departed the Sunshot Campaign. —>There have been no sightings of her since.
You shouldn’t have had to find out like this.
I know Wu Xue and Wu Tao weren’t who they said they were, and I assume you’re the same. But Yunmeng Jiang owes you a debt for sheltering and saving our people, even if you never took up a sword in our name. Most people didn’t. It’s not something I can ask from non-cultivators or foreigners. Not ever, but especially not now.
People who’ve fought with Wu Xue are sure she’s not dead. But she’s gone right for the heart of Nightless City, so there’s no proof I can give you now. The next time I’d expect to see her is after the dust settles and the blood’s dried. If at all.
If you want to send any messages, I can keep them until I see her again. Send whatever you want directly to me. My mail isn’t inspected.
.Obito: "Kei...go. Please...don't let him die. He needs to...to keep his promise."