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Yes, she's the fox that leads Yone (or whoever that is) through the afterlife.
Edit: I think /u/TyKindred is right in that this video is actually a continuation of the Ionian legend in Perennial:
Long ago, the world was perfectly balanced. It was as an enormous tree full of life, with each branch, each leaf, each bloom carefully and thoughtfully positioned so that the sun and rain could nourish them all. The people, the animals, and the spirits were at peace. There was no word for “war” because there had never been battles or bloodshed.
One day, the Gatekeeper and the Collector crossed paths. The Collector saw how many spirits the Gatekeeper had led through the spirit realm to peace and happiness, and he grew jealous of her—
The Collector grew envious of all the spirits that the Gatekeeper had helped find peace, and so he devised a plan. He took two of his strongest, loudest bells and melted them down. Then, over twelve nights, he hammered them into two blades. Into the first, he poured some of his Jealousy. Into the second, he poured some of his Obsession. Then, when spring began, he let the spirits of those swords bloom in the physical realm, and the swords grew from the ground like saplings.
Saplings. That was what the two Brothers thought the blades were when they stumbled across them in the forest.
The Brothers were the best of friends, perfectly loyal to one another and understanding of their roles in the world. The Elder would one day inherit their father’s own famed sword and lands, while the Younger would inherit their father’s ship. Both believed they would be great heroes, one at home and one abroad. One spring, they found the two sword-saplings growing in the forest. Neither of the Brothers had ever seen a tree grow so shiny, or so sharp. Together, they chopped them down, each shouldering one to bring back to their home.
Little did they know that this would be the last thing they would ever do together as Brothers while they remained alive. For as they walked home, the strange sap from the swords began to flow onto their necks, filling them with horrible thoughts and feelings… those of the Collector. Though they did not become enemies that day, they would eventually bring those blades together, a clanging of bells that would sound throughout the physical and spirit realms as nothing had before.
It's quite possible that what we witness in this video is the elder brother (represented by Spirit Blossom Yone) traversing through the afterlife after being slain by his younger brother (represented by Spirit Blossom Yasuo). For a moment after his death the elder brother is at peace, but the corruption of The Collector lures him in and (at least from what I can tell) attempts to overwhelm him. The elder brother overcomes the corruption and defeats The Collector, but not before The Collector manages to get in one final blow...
After that we cut to Yone in the physical realm, a literal manisfestation of the legend you see above. If I had to guess, the legend is an allusion to what happened to Yone: He was at peace and ready to move on after being slain by Yasuo, but the darker emotions in his soul were given strength by the growing imbalance in Ionia—thanks, Noxus—and managed to wrench him back into the physical realm as a twisted spirit seeking revenge, absolution, perhaps even both. Long story short: Perennial's Ionian legend shows us what happened to Yone in the Spirit Realm after he died, albeit through an allusion.