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Original Description:

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 antisocial, 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).

Sunset Shimmer also returned to Equestria after graduating and realizing she’s the one causing all the magic imbalances in the human world. (This was actually a thing that was planned for a special that to end up not getting made for EQG, minus returning to Equestria part, and apparently her human counterpart is involved somehow.) And lives with Twilight and helping out at the Friendship School. Through his interactions with them, Dust ends up developing crushes on both of them also at some point.

Dust’s talent in more detail.
Realizing that Size Magic.could be potentially overpowered. I worked out some stipulations for it.

Dust’ Size Magic works easier and use less mana on something he’s ‘attuned’ to for a long time such as himself, clothes, his sword, friends, ecc.

Attuning to is basically just being becoming familiar with the individual or object. Like being exposed to something ‘aura’ for a particular amount of time.
When he use it on something he’s not attuned to, such as a random pony, a size spell on them, will use up a lot larger large chunk of mana or possibly even over tax him if used repeatedly in a short time. (although inanimate objects uses less mana than living things) Also making something bigger wears him out faster than something smaller.

He has magical stamina on the part of normal ponies, so he’s not like Twilight or Starlight. (And remember Unicorn Twilight nearly knocked herself out transform Rarity to have butterfly wings that one time.), so he can run out of mana pretty fast if he’s not pacing himself, but repeated training throughout years can possibly lessen the strain on his mara reserves.