Spike is a male dragon who has been Twilight's closest friend and companion for nearly all of her life, ever since she hatched his egg during an unexpected magical surge when she was testing to get into Princess Celestia's School for Gifted Unicorns over 50 years ago.

Spike's role in this particular tale is very brief, but don't underestimate his influence. I imagine him having a gruff but kind demeanor, with a slightly deep, slightly growly voice (as befitting his draconic heritage). I am not looking for Smaug imitations.