Backstory short version.
Dust is basically a self insert OC serving as a bodyguard for Twilight and have a massive crush on her…. And Tempest, who is his Superior… And Sunset when she comes and visits…it’s complicated.

Backstory long version.
Dust is a slightly asocial, kind-hearted, bisexual Unicorn Stallion with a sense of adventure and a strong loyalty to his friends.

Despite his age, Dust hadn’t discovered his Cutie Mark and uses the excuse of becoming a Sell Sword to travel around to try different things in hope to find his talent. At some point, he got hired to recover some old texts rumored to be hidden away in some secret compartments in the Castle of the Two Sisters by a History Researcher. Upon reaching the old castle and searching it thoroughly, Dust ends up running across an old tome on Size Magic and seems to be able to understand it easily, despite not being all that good at Magic in general.

Having been always curious on what it would it be like to experience the world from a different perspective, Dust tries out a Shrinking Spell finding it pretty easy to do for him. In his excitement he ends up running out of mana from rapidly shrinking and growing repeatedly, getting stuck at really tiny size and ironically earns his cutie mark at the same time. After sometime having to survive and fight off the dangers of being tiny in a really old castle aka cockroaches, spiders, rats, bats, a snake, Tiny Dust is eventually found by Twilight, who heard about a pony inquiring about the castle and then seen entering the Everfree from town ponies.
Twilight in her investigation finding the shrunken unicorn and returns to Ponyville with him in hoof, promising to taking care of him until she’s able to fix him or he recovers enough mana to able to restore himself to proper size.

Upon growing back to normal, Twilight was curious about Dust’s innate talent with Size Magic, helps him refine his control over it and does experiments on his capabilities with it. Dust ends up developing a crush on her during this time, And offers his services to Twilight, who’s come to enjoy his company at this point, and agrees to the offer allowing Dust becoming her personal bodyguard.

Later on after returning from her penance quest, Tempest Shadow is talked into settling down at the Friendship Castle by Twilight and becomes the head of Twilight’s guard, (aka just Dust).