In my head Romilda was meant to be an Aries, but since I put her birthday at the beginning of the month rather than at the end, she's a Pisces. So she's now the sign that fits her least well, haha.
But like with Susan, I can pick out some traits that fit her: "gullibility, impracticality, emotional game-playing, delusion". I'd also agree that she's "romantic to an extreme" and that she does "spend more time daydreaming than doing her homework, to the vexation of her professors". On the whole, though, she's totally not a Pisces.
As for believing in her star sign? Well, not really, but she likes reading her horoscope, especially if it predicts good things! She's also used it as an excuse in the past: "But we had an argument because my horoscope said today would be a day for conflict! It, like, wasn't actually my fault." So while she doesn't believe in astrology for the most part, she's not above using it to her advantage.