Crabgrass

Crabgrass is an elderly, planttype monster. She works for the human Van Helscream and assists him in his mission to keep monsters at war with one another, so that they won't have the time or energy to harm humans. Crabgrass seems to be able to produce a powder that allows her to take on the appearance of anyone of her choosing, but it's also possible Van Helscream provides her with the powder.

In the webisodes, she is voiced by Wendee Lee.

Personality
Crabgrass is a cranky elderly lady with little patience for anyone but her boss. She shows a small level of disdain for the monsters around her, which is reflected in situations such as how she never bothered to learn Frankie's name, instead calling her "Francie". Like Van Helscream, Crabgrass has no faith in monsters' capability of living along with humans in peace.

Friends
Ms Crabgrass is the faithful assistant of Van Helscream, a human expert on monsters who strives to keep the monsters enemies of one another to distract them from attacking humans. Despite that Crabgrass is a monster herself, she shares his ideas and doesn't think twice about following his orders. That said, neither she nor Van Helscream would give their life for the other.

Fright On!
Crabgrass got employed at Monster High at some point in time, presumably to keep an eye on the proceedings at the one all-monster school in existence. When she heard of the plan for Monster High to share its ideals of monster-human unity with Belfry Prep and Crestcent Moon High, she became extra alert. Once it had been established that Monster High's ideals could bring vampires and werewolves together, she contacted Van Helscream so he could take care of matters personally.

Van Helscream soon arrived, and with Crabgrass's assistance, he turned Headless Headmistress Bloodgood into a statue, upon which Crabgrass assumed the headmistress's identity. The first thing she did was disbanding Frankie Stein's welcoming comittee, after which she gave Van Helscream the freedom to make his home in Bloodgood's office and manipulate the student body back into prejudicial thoughts. When Frankie noticed Van Helscream was ruining everything that had been accomplished, she went to Bloodgood's office, where she questioned Van Helscream's methods to Crabgrass. Crabgrass parried that a youth like her couldn't understand a monster expert like Van Helscream's methods and then chased her and Abbey Bominable out. However, because she had consistently referred to Frankie as "Francie" and because it was clear Nightmare was terrified of her, Frankie realized she had not been speaking to the real Bloodgood.

When later she and Lagoona Blue saw Bloodgood drink with her finger, the rest of the ghoulfriends began to believe Frankie's assessment. Upon the following question where the real Bloodgood was, Van Helscream uncovered a new statue in front of the school, the real Bloodgood. Frankie and Abbey tried to free her from the spell, but in the process got caught with her, Draculaura and Howleen Wolf by Van Helscream. The group managed to escape their predicament only just in time to prevent a werewolf-vampire fight over the missing Draculaura and Howleen, explaining to the other student that they had been manipulated. To thwart Van Helscream's plans, the students quickly arranged for a grand party that evening, so that when the skullastic superintendents would arrive, they'd see that different kinds of monsters could get along. Crabgrass and Van Helscream were left stupefied at the sudden recovery of community spirit among the student body, leaving them open for an attack by those same students. Though Van Helscream possessed the means to fight them all off, he and Crabgrass were caught offguard by Deuce's petrification beam, leaving them statues just as they had done to Bloodgood.

Trivia

 * Crabgrass's name and plantlike appearance are based on crabgrass. Crabgrass is a type of grass often classified as a weed because it thrives well in a garden not kept in prime condition and pushes away the prettier domesticated grass. Crabgrass also dies off in the fall and winter, leaving an ugly bare spot where domesticated grass would not have. But while a pest to many gardeners, crabgrass itself isn't a monster or ever associated with the macabre. It is possible the writers intended to associate her with the hungry grass, cursed grass that according to Irish legends forces starvation upon anyone who steps on it, but the only thing hinting in that direction is Nightmare's desire to eat Crabgrass.
 * Deuce's petrification usually lasts about 24 hours, though everybody seems to think it will last longer for Crabgrass and Van Helscream. Whether the two will return in future seasons has yet to be seen.