To Arnold, Hilda is the opposite version of the Helga he knows, when in truth, Hilda is the kind, sophisticated and poetic side of Helga that Arnold rarely, if ever, gets to see. She is just Helga without the mean front she puts up, so essentailly the Helga deep down: calm, kind, sweet, helpful, polite, friendly, bubbly, poetic, and emotional.
Hilda is seen in similar clothes to Helga, but with slight differences. Hilda wears a short pink jumpsuit with one red stripe through the middle the same as Helga's dress. They both have white shoes and white short-sleeved undershirts, but Hilda's has a frilly collar. Hilda has longer, fuller bangs that fall over her unibrow, and her hair is up in a ponytail rather than pigtails and her blonde hair appears to be a slightly more pale color. She also wears a pink scrunchy instead of a bow. The only parts of Hilda's outfit that have no correlation to Helga are her blue socks and round red stud earrings.
She appears in Arnold's nightmare, where almost everything is the exact opposite. To Arnold, Hilda is the opposite version of the Helga he knows, when in truth, Hilda is the kind, sophisticated and poetic side of Helga that Arnold rarely, if ever, gets to see.
It is significant to note that Arnold, as in previous episodes, likes this version of Helga. Due to it being a dream, Arnold gets a crush on Hilda; however, because of the opposite universe of the nightmare, Hilda likes Arnie, Arnold's cousin and tries to get her attention but his heart is broken, even in a dream, yet again.