As Gendry was Arya’s only remaining true friend, Beric landed himself a spot on Arya’s list.
In the grand scheme of Game of Thrones, Beric Dondarron isn’t exactly the biggest character ever to hit the airwaves.
Although he seemed like an ally to Arya and Co. early on, he earned himself a place on her kill list when he and the Brotherhood sold Gendry to Melisandre.
Nevertheless, he was a key part of the plot of Season 3 as a member of the Brotherhood Without Banners.