Neil Gaiman describes the flea's original form as something like a decaying circus tent:
I thought I was looking at a building at first: that it was some kind of tent, as high as a country church, made of gray and pink canvas that flapped in the gusts of storm wind, in that orange sky: a lopsided canvas structure aged by weather and ripped by time. […] Its face was ragged, and its eyes were deep holes in the fabric. There was nothing behind it, just a gray canvas mask, huger than I could have imagined, all ripped and torn, blowing in the gusts of storm wind. (4.56)
It's kind of an abstract form, so she's much easier to imagine as Ursula Monkton: the prim, tidy boarder who seduces the boy's father and enchants his sister.
Appearances aside though, she's quite the piece of work. Ursula doesn't want to disrupt the lives of the people she meets, and she's not trying to be a malicious harpy. To the contrary—her motivation is to make everyone happy:
"She finds what they think they need and she tries to give it to them. She's doing it to make the world into something she'll be happier in. Somewhere more comfortable for her. Somewhere cleaner." (10.88)
The only problem, of course, is that she doesn't understand how to do it properly. In the beginning, Ursula senses all of the money troubles in the boy's neighborhood, and so she tries to give everyone money. But as we all know: Mo' money, mo' problems—you know, like waking up choking on a vintage coin.
Ursula Monkton is a simple creature. Gran calls her kind "fleas" because they're typically harmless, and a little stupid. Unfortunately though, her simplicity makes her a lot like a petulant toddler: everything is black or white, good or bad, happy or sad. If you thwart her plans she gets nasty real quickly—so when Lettie and the boy come to rain on her parade and send her home, she's no longer concerned with making people happy. Now she's prepared to do some really nasty things in order to stay in the little part of the world she's found:
"I need the boy safe. I promised I'd keep him in the attic, so the attic it shall be. But you, little farm-girl. What shall I do with you? Something appropriate. Perhaps I ought to turn you inside out, so your heart and brains and flesh are all naked and exposed on the outside, and the skin-side's inside. Then I'll keep you wrapped up in my room here, with your eyes staring forever at the darkness inside yourself. I can do that." (10.108)
Yeah, we're pretty sure that wouldn't make Lettie happy. It sounds miserable. Add those threats to what she actually does to the poor boy (getting drowned in the bathtub by your own father isn't exactly joyful) and we've got ourselves a villain. So even though she might've started out as a relatively benign being (try running that phrase through autocorrect a few times), Ursula Monkton is ultimately only looking after her own happiness—and that means she's definitely not one of the good guys.