Ulti doesn't initially react, but her brother is enraged and makes to attack Izuku, only for Ulti to punch him into the ceiling and squeal that Izuku is cute before glomping him.
When Page One berates his sister for grabbing a boyfriend so rapidly, Ulti rhetorically asks if she prevents Page One from chasing skirts—to which her brother angrily responds that she does do that.
Despite having only just met him, Ulti fantasizes about all sorts of things to do with Izuku, even declaring at one point that they're going to get married and have kids together.
When asked why she would grab some random kid, Ulti tactlessly points to every other male present and points out their respective negative traits that make them unworthy of her, ignoring all of their offended remarks. The last guy doesn't even get an explanation beyond a disappointed sigh, much to his indignation.
To give Izuku a way out from the clingy Ulti, Yamato asks WWOD (What Would Oden Do?). Well, if Yamato’s next actions are any indication, Oden would strip down to his fundoshi, do a mad dance like during his years of imposed self-humiliation, and start a drunken brawl.
Ulti is pissed after Yamato escapes (in a smokescreen Izuku provided after an exchange blew up dust), but forgets about that in favor of wailing about how Izuku vanished as well. At least until her brain latches onto the idea that Yamato kidnapped Izuku to ravish him.
When Izuku is told they’re going to meet the King of Doyle, he freezes up so badly that Yamato has to carry him.
While staying at Reichen Bach's palace, Izuku and Yamato are brought breakfast. Yamato blows on some of her food to cool it down, but unbeknownst to her her ice powers from her Devil Fruit kick in and the porridge ends up stone cold. From her pout, this is not the first time she's accidentally ruined her food in this manner.
The next day, Izuku and Yamato go clothes shopping. The seamstresses they meet are ecstatic to have customers and immediately start taking measurements... on Izuku, mistaking him for an okama given his rather cute build.
Not long after, Yamato gets her first fitting and asks Izuku what he thinks. His response that she looks great is heartwarming, but swings to hilarity when Yamato lifts the skirt and flashes him (with the Vestiges pointing out that she's in Victorian-era bloomers and his classmates wore far less clothing than that). When this gets him to seize up again, Ensign Doll snarks that Izuku needs to go outside more.
When fighting Handsome, Izuku snaps at him, calling him out for how stupid he was for being a pirate and eating a Devil Fruit that basically made him allergic to water.
What ends up causing Yamato the most pain during the Doyle arc? Blazing Inferno Hellfire Sauce she eats during the celebratory ball/banquet, causing her to run about the hall wailing in pain while Izuku follows her to find some milk.
The author posts a size chart that shows just how much taller than the normal-sized Ulti Yamato is. He states that Izuku flying Yamato around is like her having a jet-pack, that attaches to the front like a baby carrier.
With Izuku out of her grasp, Ulti decides to make Page One her "cuddle practice", much to his dismay. Then there is her Missing Steps Plan to convince Kaido to spare Izuku and have Queen fix him up with cybernetics once Kaido's done ripping him to pieces, all of which Page One objects to.
On their way to Sphinx, Yamato and Whitey Bay get into a childish argument over Yamato not thinking Bay is scary, complete with "Are not! Are too!" back and forth.
Izuku gets a taste of One Piece weirdness when encountering sphinx that can only say "Pork Ramen" and is left absolutely baffled.
During their talk, Whitebeard asks Izuku what kind of man All Might was. Izuku misses the specific wording and launches into fanboy babble about all of All Might's cool moves and outfits, and almost immediately gets the "Whitebeard is annoyed" look from the Emperor. Then Yamato starts a similar babble about Oden, and Whitebeard becomes even more annoyed.
Uta starts openly fantasizing about Izuku as a captain of his own ship, with leather pants and an open shirt showing off his abs and Uta in his arms, flustering Izuku even more.
Marco walks in on Izuku and Yamato after they've finished with Uta, and thanks to Yamato having no filter and talking about "big ones and small ones" (forgetting to mention she's talking about boats) he thinks they've had a Relationship Upgrade and offers to talk to Izuku about things, flustering Izuku more.
Izuku belts out that it’s Not What It Looks Like, at which point Marco smirks, making it clear that he understands exactly what Izuku means, but he’s going to tease them mercilessly anyway.
Teach enters, drawn by the noise. He's in a speedo. This is the breaking point and Izuku throws everyone out with Black Whip.
After all this, Izuku wonders what other weird things he'll meet on his trip, like a talking skeleton or a demon racoon dog. Daigoro starts to ask if he just jinxed them before the other Vestiges beat him up.
Before things take a turn for the worst, the Whitebeard Pirates are happily teasing Izuku and Yamato about their future "love boat" the next day at dinner; Izuku looks about ready to bury himself in the ground, while Yamato is confused by the whole thing.
Izuku, Yamato, and Reiju encounter the Caribou pirates on their first stop at a populated island in the North Blue. Things go poorly... for Caribou:
His attempt at being intimidating/"attractive" get shut down by Reiju telling him he's not cute. Coribou unhelpfully repeats that, to his annoyance.
While he's boasting about his Swamp-Swamp Fruit's powers (and once again being annoyed by Coribou explaining his powers to the enemy), Yamato interrupts him with a Haki-infused punch to the face, sending him flying out the window. Everyone, including Reiju, is stunned to silence.
Outside now, Caribou tries to intimidate Yamato with his impressive bounty of... Five million berries. Even Izuku can't help but bust a gut at how pathetically tiny that is compared to New World Bounties.
The Third Wielder (Bruce) once again stands by his declaration that Yamato is a dumbass when she says Trafalgar Law "seems nice" after they watch him abuse and humiliate the pirates he just beat.
While discussing his comic book collection, Law lays down one rule, a rule so important and vital to their continued relationship that breaking it will will be completely unforgivable: Don't crease the pages and use proper bookmarks when reading.
After body slamming Ace in his dinosaur form, Queen gets up and starts gloating... only to realize half of his audience has been knocked out cold thanks to Ace's Conqueror's Haki, undercutting his attempts to look cool.
Yamato is caught off guard by Law's declaration that he hates bread, shoving a forkful of waffles in his face to try and convince him much to his annoyance.
