My first OC named Dust. Cutie mark is up and down arrows signifying his talent for Size Magic.

[Dust without his attire](/images/3116037)

Backstory short version.
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 Unicorn Stallion with a sense of adventure and a strong loyalty to his friends.

Despite his age Dust hadn't discovered his Cutie Mark, using the excuse of becoming a Sell Sword to travel around and try different things to hopefully 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 and and when reaching the castle ends up running across an old tome on Size Magic and seems to understand it easily, despite not being all that good at Magic.
Having been always curious on would it be like to experience the world from a different perspective, tries it out finding it pretty easy to do for him and in his excitement 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. And after having to fight off the dangers of being tiny in a really old castle aka spiders, rats, bats, a snake is eventually found by Twilight who heard about a pony inquiring about the castle and and is seen entering the Everfree from town ponies, and goes to investigate, finding the shrunken Dust and returns to Ponyville, taking care of him until he recovers enough able to restore himself to proper size.

Twilight curious about his innate talent with Size Magic, helps him refine his control over it and does experiments on his capabilities with it. Dust developed crush on her, and offers his services to Twilight, who’s become to enjoy his company at this point in agreed and Dust becoming her bodyguard.

Later on Tempest Shadow is talked into staying at the Friendship Castle by Twilight and becomes the head of Twilight’s guard, (aka just Dust) and Sunset 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 and Dust ends up developing crushes on both of them also.

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 and 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 could use up a large chunk of mana or possibly over tax him. (although inanimate objects are easier than living things) 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 a mana pretty fast. Also making something bigger wears him out faster than something smaller.