Just pretend like nothing's wrong, and he won't notice anything.
Faerie just wanted to see what being a guardspony was like, it's not his fault his brother decided to have a shift right then. Luckily, Faerie has a great plan on how to avoid being caught! Everyone knows the guard's armour has a mild illusion enchantment on them to make them look the same, so all he has to do is stand in attention and Lumi won't notice a thing.
Unfortunetly for Faerie, A) it needs to be activated, B) the armor is keyed to the pony it's for and C) it only changes their color, so Luminous would have seen an extremely small, alicorn guard if it had worked. Sorry bud.
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Introducing the last of the ShiningDance kiddos; Luminous Spirit. A rigorous, no-nonsense pony, who prefers the company of himself to anyone else. He is also very honest, honorable and dedicated to the empire, especially on keeping its citizens safe. He's currently a member of the crystal guard, and will most likely take over the position of its captain once Shining steps down.
He's a bit awkward, and doesn't know how to deal with other ponies outside of the context of the guard. As the middle child between the heir of the throne and the twins who stuck together, he often felt a bit lonely and forgotten. When Fire Aurora was born and became the baby of the family, he very much resented her for it, and would pick on and bully her. As and adult, this is something he deeply regrets. He's not sure how to fix it though; he's tried to apologisein his own stilted, monotone wayand Aurora kinda just shrugged it of, but still avoided him. He doesn't quite know where to go from there, so he usually just gives her space and tries not to bother her.
The one good thing that came from it was his determination to never hurt anyone like that again; always doing his best to not judge others, help people in need, and stand up against injustice. Even if it's not immediately obvious with his stoic nature.