spoilers for the end of undertale yellow's no mercy route, neutral route, and major parts of pacifist. please do not read if you do not wish to be spoiled. you have been warned.
i apologize for any possible inaccuracies. i tried to be as accurate as possible by rewatching cutscenes, but there's always a chance i got things wrong. it also might be a bit rambly because i wrote them as messages. still, i have a lot of thoughts on this decision undertale yellow made, which i'd like to believe i backed with solid evidence. and in the end, it is only my opinion. i'd just like to have my point of view seen.
so, in regards to martlet and undertale yellow's no mercy route:
i think martlet is the perfect choice as no mercy's last stand, actually.
in undertale, it's sans, and yeah, he shows that he's menacing no matter what route you're on, but for the most part he comes off as a goofy slacker with some mysteries.
it's the fact that the player drives him to the state where he has to do something, because they're on a killing spree with nobody spared, literally. and he reveals that he's actually been working hard on all the time anomaly shenanigans behind the scenes that make sans such a good character to be the last defense between the player and asgore (until flowey kills him, and then the player kills flowey, and then chara kills the player...)
with undertale yellow, martlet is a very optimistic and genuine person. she's a bit of an airhead, but she still tries her best as a member of the royal guard. all the other major bosses fight clover in pacifist for reasons such as mistaking clover for the blue soul believing to be tracked down by them and defending themselves (dalv), blaming clover for their ruined relationships and believing that bringing in their soul themselves will repair their reputation (starlo), and of course, requiring clover's soul in a last-ditch effort to save their daughter from being at death's door (ceroba).
martlet is literally going by protocol. she's following the rules to the letter, and gets confused when clover refuses to attack, not even doing so herself before she gets to the part where she's allowed to. and the part where clover insults her behavior as a royal guard is where she falters and decides to spare clover, because it's true. her best isn't enough for them. the royal guard is under asgore's rule to take human's souls for monsterkind, but that isn't enough for her to finish the job.
this is where you keep in mind martlet and chujin's conversation that clover sees in the no mercy route. her mentor, that she admired and was close to, is extremely cautious of humans due to the incident with the blue soul. but even after knowing the damage they can cause, she has hope that not all of them are bad, as she'll say in no mercy herself.
so that's why in no mercy, when clover is delivering their twisted sense of justice and killing every monster they can, she knows she has to stop them. in the first encounter, after she realizes clover is the culprit, she tosses the guide book and puts everything she's got into the fight. but that, just as well, isn't enough. she has to retreat because they're too powerful, even though clover could continue their rampage. but still, after almost dying to them, she tries to convince them to end the violence because she wants to believe that they have an ounce of sympathy for their victims.
if no mercy is aborted, when clover meets her on the roof, she throws away the determination serum she picked up from the lab just like in regular neutral. because she sees that, at least in one instance, they didn't choose murder. and that's good enough for her. that's all it takes, even if flowey goes in for the kill and martlet blames clover. she only goes so far to do so in her dying moments, and because flowey purposefully hid in the shadows, so how would she know it wasn't them?
when no mercy is fully initiated, she doesn't even instigate the combat; clover does. they're so trigger-happy that they waste no time. and at that point she has to use the last weapon monsterkind has to prevent clover from reaching asgore. she waited until the last moment, hoping clover would change their mind. but when there's no hope for them, she harnesses that determination to become the most powerful monster in the underground, the zenith of monsterkind. she'll wield the determination to deliver justice unto clover.
