This is Bright Spark, he is the son of EquestriaGirl’s Twilight and Timber Spruce. He’s a deer because Assassin-or-Shadow and I headcannon Timber Spruce to be a deer.
 
He grew up hearing tales of his parent’s adventures and seeing magic in action. He LOVES magic, he’s so fascinated by it and dreams of seeing Equestria and learning more about magic.
 
One day he goes digging through his parent’s stuff, finding Twilight’s old research papers, and discovers the entrance to Equestia. He runs off before anyone can stop him, leaving his parents in a state trying to figure out where their son has disappeared. Once in Equestria it’s not long before he’s found, considering the mirror is in Twi’s library. It takes some convincing, but finally his family lets him stay with Princess Twilight and her family for a little while until school starts up again.
 
Candlewick is not happy about this. Bright Spark is excited about staying with them and starts referring to “Wickster” as his brother. (Candlewick hates the nickname). As payback he calls him “Sparky”. It doesn’t have quite the intended effect since Bright Spark thinks it’s cool. (That’s a lie, he hates it, but Bright Spark tries to hide it. Similar to his Aunt Gloriosa, he puts on a smile and tries to be optimistic, even if he’s one more annoyance away from blowing). However, they do butt-heads occasionally when it comes to magic and science.
 
Candlewick is admant that Science is superior to Magic. Science exists all the time, it can’t be stolen and taken away like magic can. And there’s plenty that science can do without resorting to magic.  
Bright Spark says Magic is better. It can do near impossible things, breaking reality, and is just so much more interesting. More powerful.
 
(I find this hilarious because Candlewick is a chaos being able to control forces of magic most - if not all - unicorns could only dream of controlling, but he never uses these abilities (doesn’t care for them). And Bright Spark has no magical abilities being a deer and all, but dreams of being able to use the kind of magic Candlewick was born with)
 
So yeah, their differing opinions, and Candlewick’s obvious dislike of Bright Spark can cause quite the number of arguments. Bright Spark tries to be civil most of the time, but hates it when Wickster bags on magic.
 
Other than that, he’s a sweet curious boy who loves how mundane everyone in equestria views magic. It’s just so amazing to him.