Kei during the majority of the Rashomon Arc. When she manages to keep Isobu's influence at bay, fight off the Puppet-Master Seal, and battle Kiri ANBU alongside her teammates - see Underestimating Badassery on the main page.
Kei gets another when she manages to convince Yang Kurama to let himself get sealed into Naruto, earning her the respect of Jiraiya and Tsunade. Later turns out to be a dual CMOA, because of later revelations that Isobu had a significant hand in Kurama's decision to play nice, not that Kei or anyone else could hear what he said.
Rin gets one during the Commencement Arc, too. At Minato's prompting, she throws herself into the path of Black Zetsu-possessed Obito as a distraction, despite barely any assurance that she'd survive.
In one of the side-story chapters, we get to see Kei's mother's backstory. Originally born to a samurai clan in the Land of Iron, the then-Tomoe Uesugi survives her clan being massacred on her wedding night, managing to only rescue her youngest nephew. What does she do next? Why, spirit her nephew someplace safe first. Then she spends the next six months hunting down everyone remotely related to said massacre and killing every last one of them in a completely successful assassination campaign, of course. They didn't call her the Ghoul of Three Wolves for nothing.
In the fight between Kei and Minato, awesome moments abound:
Kei manages to shut down her opponent's mobility advantage thanks to some well-crafted Tailed Beast powers, screwing up his sensing ability.
Kei and Isobu figure out how to combine their respective skill-sets, resulting in Isobu being able to use the Rasengan with his tails, Kei using kenjutsu in a Version 2 Chakra Cloak, cutting her way out of the Toad Mouth Bind jutsu, and eventually in a Tailed Beast Dual Wielding swords made of coral against Gamabunta.
Minato never once loses his cool over the course of the fight, simply pulling out more and more tricks from his figurative hat the more that Kei escalates. Nothing throws him off his game so badly that he can't recover almost instantly, which eventually gets him the win.
Nagato, taking up Kei's half-sarcastic idea, launches the Demonic Statue of the Outer Path into low earth orbit.
And in the next chapter, he conducts Tailed Beast chakra through his body and survives, by rapidly converting it to an electric charge and sending it straight into the ground.
Hell, he even gets the actual killing blow, taking Orochimaru's head off with his first offensive application of Kamui and seemingly killing the nigh-immortal Sannin off for good.
Kei, during the Memory Arc when she manages to kill Orochimaru five to six times over, some of them on her own. And before that point, she effectively kills half of the Sorayama invasion force, including a chakra vampire kunoichi who had already killed most of the Chinatsugumi singlehandedly.
Gai's repeated Big Damn Heroes moments in the Memory Arc, where, in rapid succession, he kills the leader of the Kirigakure team, mission-kills Yagura by breaking his spine, rescues his downed teammate the second he has the opportunity, and takes on Manda with the Morning Peacock taijutsu skill.
Kushina, when she jumps out of Kamui in a six-tailed V2 Tailed Beast cloak and tears right into Orochimaru.
This one technically counts as an Off Screen Moment Of Awesome, but when Kushina fights Orochimaru, she nearly obliterates the entirety of the landscape surrounding them, and she leaves said fight without so much as a scratch.
In the "Cold-Calling" short story during the The B-Plot, the reader finds out that the battle still left visible (and baffling, to outsiders) scars on the landscape still visible almost a decade later.
Ocean Stars Falling
In her first two weeks on the Moby Dick, Kei saves Thatch's life when Teach tries to kill him and steal the Yami Yami no Mi, though the latter gets away and kicks off Ace's subplot.
And all she used to do it was a single kunai.
Because she did so, Thatch later gets the Yami Yami no Mi back (thanks to Marco's luck) and is able to wield its powers, after Teach gets killed in a Team Jinchuriki ambush and the fruit reincarnates. And later still, this means Thatch can go toe to toe with Admiral light-based powers.
While both Kei and Ace were sandbagging in hindsight, the two of them have an exhibition match that demonstrates just how dangerous a 26-year-old veteran jinchuriki can be in a combat scenario.
After the disaster that was their first attempt to take on the Blackbeard Pirates (mostly due to lack of coordination), Kei and Yugito grab their newest recruit to Team Jinchuriki and press-gang him into helping them actually use shinobi tactics on unsuspecting pirates. Utakata agrees, though grudgingly. For the Blackbeard Pirates, it's a Total Party Kill.
Saiken and Utakata clog the ocean with slime to keep the pirates from escaping, while Isobu weaves notice-me-not genjutsu.
Thanks to Yugito's fire powers, the ship itself is burned to cinders despite some frantic attempts to put it out.
Kei presses the advantage afforded by the distraction and infiltrates the ship, only to find that Ace has already been passed off to the Impel Down jailors. At that point, she changes the mission parameters from "rescue" to "kill everyone."
Yugito sets two of Teach's crewmates on fire from the word go, and refuses to let the fire burn out until they're dead.
Team Jinchuriki decide that, because obviously Ace still needs rescuing and he's in Impel Down, it's time to stage a prison break. They pause exactly long enough to get directions from the Whitebeard Pirates, then it's off to see the wizard.
The Straw Hat Pirates manage to avoid being scrambled by Kuma, so all of them show up to the Impel Down fight just as Luffy would have done if he was alone in canon. With all of them, though, it's Plot Tailored to the Party time, though nobody involved was aware before going inside.
Gaara trivializes the first floor of Impel Down, because his sand means the razor-edged plants and other hazards mean nothing to him.
Level two? Stomped on by the Straw Hats with minimal assistance from anyone else, though Utakata knocks a hole through a wall to take out a guard station and steal all their snails.
Yugito beats down the Jailer Beast on Level Three with its own club.
Utakata takes on Magellan on the next floor and, in the time it takes for the rest of the party to advance all the way to Level Six, outright kills him (and, accidentally, nearly everyone else on Level Four because Magellan's powers don't account for allies).
Chopper and Brook have zero problems with Level Five, being respectively a reindeer and a skeleton with no relevant nerve endings in the cold.
Kei rips the sea prism stone front off of Ace's cell when she and Luffy finally make it through Level Six. Say what you will about her lack of ambition or worrywart nature, but Kei doesn't fuck around when protecting her friends.
For all of their decisions in their piratical debut, Team Jinchuriki's adult (human) members finally earn bounties from the World Government. The lowest of them belongs to "Cobalt Lioness" Yugito (355 million beris), the middle one to "Tidal Blade" Kei (430 million beris), and the highest to "Carnation Prince" Utakata (575 million). The last of them is twenty-five million beris higher than Ace's current bounty. Not bad for a bunch of newcomers.
The implication afterward is that the only reason their respective bounties aren't significantly higher is because the World Government is trying to downplay the fact that their Hellhole Prison doesn't exist anymore. Even Boa Hancock didn't actually know the extent of the damage done before confronting Team Jinchuriki and the Straw Hat Pirates.
Kei utterly destroys twenty-seven of the robot opponents in the UA Entrance Exam, apparently solely so no one would dare underestimate her "Quirk" again. It helps maintain her cover and means Isobu gets something akin to exercise!
Despite the ban on using Quirks in public, Hitoshi ends up being the one to solve the problem of the Nomu chase by calling Eraserhead to their location.
In the next chapter, Kei does adhere to the laws by a technicality—because only heroes are allowed to attack using their Quirks, she deflects all of Stain's thrown knives almost barehanded while Shoto Todoroki keeps Stain at a distance with his powers instead. Because the two of them are defending a paralyzed Obito, the impetus to retreat falls instead on Stain. He doesn't take it that chance, and ends up stuck to the alley wall via Water Prison and then getting frozen there. Since they didn't beat him into the ground, the kids walk away scot-free and Stain ends up in Tartarus.
Kei's team take up the Mysterious Protector role during the Camp Arc. This means mainly that they end up getting into more fights than they have in months, but the fights are pretty great.
Kei slams into Muscular while chasing down Izuku, resulting in the guy getting the Curb-Stomp Battle treatment the USJ Nomu avoided thanks to All Might. She cuts him down to size in three moves.
Obito's Kamui means that, once he stops taunting Magne and Spinner, he manages to get every kid out of the forest except for the few stuck with Kei and Kakashi. While Kakashi isn't happy to hear that Team Minato's easy out almost knocked himself unconscious playing hero, he likely mitigated most of the possible damage done to the people their team was supposed to protect. He even protected Ragdoll and Pixie-Bob by moving them out of the line of fire at the beginning of the fight, preventing most of the trouble the Pussycats suffered in the original story.
One for the League: Compress, Twice, and Kurogiri coordinate their powers properly for once, giving them just enough of a chance to snatch Bakugo out from under Kei's noise.
Midoriya manages to figure out that Kei is associated with the mysterious masked not-heroes at the training camp through circumstantial evidence from his hospital bed. It's a bit of a Bat Deduction, but within Izuku's capabilities. He even manages to convinced Hitoshi, to some degree.
Kei's thought process on the whole thing: She'll wait in the bar, tied up and surrounded by villains, until she can get her hands on All For One. If they'd taken her to the Nomu factory, she'd have killed her way out, but because Bakugo is within the blast radius, she puts it off until she can secure a better opening. There's no fear, only Kei and Isobu deciding how the future's gonna go.
Bakugo, meanwhile, thinks fast and decides to ally with Kei as soon as he realizes she's there. Even while bound hand and foot to a chair, he still decides to deliberately antagonize the League to keep their attention off Kei, and Kei likewise outright attacks Toga while bound to distract her from Bakugo.
The heroes show up to the party on schedule. By that point, Kei's already gotten free of her handcuffs and knocked Kurogiri unconscious to keep him from using his Quirk. Likewise, Bakugo's already rejected the recruitment offer and decided to go down fighting. Then All Might smashes through the wall.
To make matters that much worse for the League, Minato makes his way through to Japan from Konoha and covertly joins the heroes' rescue operations. When the attack on the bar goes sideways for the heroes and Nomu start teleporting in, he kills most of them within seconds and grabs All Might to take him to the new battle location and get this rescue mission back on track.
Kei's initial disappointment at not being able to find and bag All For One turns out to be ill-founded, because a previously unknown Quirk transports them away from the bar and to All For One's location. Which means Minato, who can sense chakra and definitely track Kei's in particular, knows exactly where All For One is.
All For One's reassuring speech to Shigaraki is terminally interrupted by Minato warping All Might directly in front of him.
Kei finally, finally gets to transform into a V2 Tailed Beast Cloak mode even with witnesses, taking out the League's junior members in seconds.
Bakugo: "Seaweed Head." V2!Kei: "Hm?" Bakugo: "Fuck them up."