Crab Weasley and Goyle Weasley, also known as Crabbe and Goyke, are friends of Ebony's who randomly show up in several scenes. They are likely based on Crabbe and Goyle, Draco's friends, from the books.
Background
In My Immortal, Crab and Goyle are members of the Weasley family. Darkness, Diabolo, Crab and Goyle all have the same father. Their father was a vampire who raped them, so they all became gothic and converted to "Stanism."
Role in the Story
The duo first appear in Chapter 17, where they set up a fireworks display obtained from from "Wesley's Whizard Wises"
They reappear inn Chapter 22, where it is mentioned that both appear to enjoy crossdressing, as it was mentioned they wore the same clothing their sister Jenny Darkness was wearing, who at the time wore a black dress.
Difference from the Original
Like in the Harry Potter series, Crab and Goyle are both Draco's friends and fellow Slytherins. However, in My Immortal they are also part of the same "goffic" clique as Draco. They have been relegated from his two main henchmen to background characters, as well.
The main difference from the original characters might be them being part of the Weasley family and siblings of Darkness and Diabolo. Whereas they had little background in Rowling's books, Tara Gilesbie gave the four siblings a tragic backstory involving parental abuse, suicide and becoming gothic.
It appears that Tara Gilesbie may have mixed up Crabbe and Goyle with the characters Fred and George Weasley from the books, as evidenced by Crab and Goyle performing actions that are more indicative to the behavior of the Weasley twins, such as setting up the fireworks.