It’s Zelda herself. Nintendo made it a point to hint that the character in the trailer wasn’t Link, and for every reveal of Zelda featuring Link as the protagonist, it always features him wielding The Master Sword and Hylian Shield. It’s only fitting that for a game featuring Zelda herself, they’d feature her signature weapons: a bow and magical arrows. It’s her weapon of choice in every single game where she has to fight. She uses them in Smash, she uses them in the upcoming Hyrule Warriors, she uses them in Wind Waker in the final battle against Ganon, etc.
The first arrows she fires are fire arrows:
But then, she switches to this:
And, those could be ice arrows, but I’m 99% sure they’re light arrows instead, and the trailer ends there because Nintendo doesn’t want to give away the effect of the arrows hitting and making it too obvious that it’s Zelda.