"Still enraged, the yeti did not expect the first snowball to the face." That is the turn in Riot's biography of Nunu & Willump: a guardian who had killed every intruder for centuries, disarmed by a child who decided he was not a monster. It is also the moment the last magic of a dead civilisation in the world of Runeterra changed hands, from a gem to a boy.
Who they are
Nunu was born to the Notai, nomads who crossed the Freljord bringing one last celebration to every village before winter. His mother Layka collected stories and turned them into songs, and on his fifth nameday she gave him a flute so he could play them himself. When raiders hit the caravan he was dragged to safety and she was not. The surviving Notai would not let a child search for her, and were not impressed when he showed them the flute he now called Svellsongur, after a mighty blade that existed only in his head. He met Lissandra, leader of the Frostguard, who asked him endless questions about his mother's songs and kept coming back to one in particular. The other children had daggers now and teased him for his flute.
So he chose a monster. The tribe whispered of a beast that killed everyone who sought its power, hunters sent every year and never returned, and one of his mother's songs would not leave his head. He would name the beast, and it would answer his challenge. He tamed a herd of elkyr with the flute and went alone into the snow.
The beast was the last magical yeti, guardian of a gem that swirled with the frozen dreams of any mortal mind nearby, left after the yeti civilisation was destroyed in a cataclysm of ice and its people stripped of their magic and reduced to savagery. He had found no worthy vessel in centuries; the men who came had only malice, so a monster met them. He had forgotten his name. When the boy stumbled in, the gem showed him heroes slaying dragons, the boy drew his flute like a sword, and then saw someone who needed a friend and threw a snowball. The yeti threw one back, grew furrier and friendlier, then broke the flute by accident, and in the boy's grief saw through the gem a caravan burning and a love fighting despair, something he had never felt from a human, not even the three sisters who had come to him long ago. He passed the magic into the child. The flute was mended in dreams that hardened into True Ice, and the boy imagined a best friend named "Willump". They roam the Freljord now, chasing his mother's songs and hoping she is out there, while the dark ice at the heart of the Freljord thaws.
Where they belong
Riot's faction pages put Nunu & Willump in the Freljord, and the biography never leaves it. No race is listed; Nunu is a human boy of the Notai, and Willump is a yeti, the last with magic.
Two champions are linked to them. Lissandra is in the biography as the Frostguard leader questioning a child about a song; the gem's visions of "betrayal by the blind one" and of three sisters are left for the reader to connect. Anivia is named once, in the frost spilling from her wings that the Notai rode.
In the Riot MMO, as a guess
This is speculation. Nunu is a child and Willump is the last of his kind, and the honest guess is that Riot keeps children out of the fighting, which makes the pair either absent or a special case: an escort quest across the Freljord in search of a lost mother, with the yeti as the muscle and the flute as the map. Neither is a boss; the boss in their story is the "monster" the hunters never returned from, and the biography has already turned him into a friend. The general argument is in will champions be playable?, and their case is the one where the answer might be neither NPC nor boss but a cutscene.
What we do not know
Whether the game's era, the open question in when is the Riot MMO set?, finds Nunu alive and travelling with Willump, still a boy in a caravan, or grown, since the biography does not date the raid.
What happened to Layka, and which song Lissandra wanted. The biography gives a voice on the wind and a Frostguard's questions, and answers neither.