Frisk, the oblivious senpai - Gallifreya
It was a joke, of sorts, amongst the students of New New Home Comprehensive School. Frisk saves... and resets to make sure things turn out all right.
But as the only Ambassador amongst the integrated students... the joke turned out to be accurate. Sometimes, chillingly so.
Human kids, allegedly present to foster a sense of belonging and understanding, tended to be horribly cruel to the monster kids at the school. Their idea of pranks... well.
They thought of things like tripping Kid the monster kid, who was armless and could not save their face from the inevitable bruises. Frisk just happened to be there to catch them in time. And didn't see the blushes that their accidental embrace and caress engendered.
They routinely tormented the Dogs, which should have got them expelled. Except that Frisk had a knack for making the Dogs forgive and forget with the aid of pets and scritches.
They gave up on attempting to torment the Cats. Cats had claws and an unrelenting enthusiasm.
But there were many others, including the ghosts, who Frisk rescued from torment or attempted murder on a regular basis.
Pirette was their favourite victim. Every day, some unthinking human would try to douse her with a bucket or a cup of water. Only to find Frisk drenched instead of the green flame girl.
It got so bad that Principal Toriel had to hold an assembly to educate the students about Firefolk and how trying to quench them was tantamount to attempted murder. Then she had to educate the parents as well. Then she had to get the authorities involved an press a court case or three before they finally got the message.
The powers of wilful ignorance are an amazing thing.
An equilibrium eventually got accomplished, mostly with Frisk thwarting any and all attempts to harm monsters. And all seemed normal.
Until the monster holiday of All Hearts Day rolled around. It was two weeks after the very similar human holiday of Valentine's day, and Frisk wasn't expecting very much when they unlocked their locker.
There was a soft pop as the extra-dimensional storage unit burst open, spilling love notes and heart-shaped boxes in a seemingly endless barrage that nearly buried Frisk in its tides. Frisk extracted themself and opened a heart-sealed envelope at random.
They sat, completely bemused, reading letter after letter. Thanks. Praise. Confessions of love. Confessions of admiration. Requests to play fetch. And enough monster treats to invigorate an army.
The eager monster crowd blocked the hallway. Trying to divine which note or gift had gained Frisk's favour. It was a fortunate thing, indeed, that any remaining bullies were too intimidated by numbers to try anything.
It was also a very fortunate thing that the Principal was also Frisk's adopted mother. Who declared a half-holiday so that they could at least tidy away the abundant tokens of adoration.
There were even more than a few from the humans at the school.
 King Asgore remains absolutely horrible at naming things.