>>282397300 (OP)
She's by far the best addition to Guts' party and you can tell Miura thought so too by sheer depth of her and Guts' development affecting each other; she also heavily affects Farnese and Isidro. Prior to her arrival the only other competent companion was Serpico and she's a cut above him too.
What, in my eyes at least, also helps her is the fact that she both explains lore as well as the fact that-her being a witch and what not-magic in Berserk is quite in-depth and interesting with Miura putting a lot of thought into the concept itself. Most magical application in other fantasy media ends up being a "it's magic, I don't have to explain shit" sort of progress enabler, magic in Berserk feels naturally embedded in the world itself. So much so in fact that it's even more interesting and well thought out to me than the whole Godhand and Maelstrom of time and all that stuff related to the Eclipse and God itself.
In short, I can't think of a better witch than Shierke in any media.