Darci Scott is one of Claire's best friends, and also the person behind the mole costume. Darci appears with Mary Wang throughout the first episodes as two of Claire's closest friends, up until the point that she begins absconding them to hang out with Jim and Toby. Unlike Mary Wang, Darci doesn't realize that Claire is now a Trollhunter, alongside Jim and Toby. She seems to support Jim and Claire getting together in episode 20, as referenced by her frequent asking if Jim asked Claire to go to the dance with him.
In episode 21 'Party Monster', they think that Claire is throwing a party without them, which was actually thrown by NotEnrique, who invited the other Trolls of Trollmarket. Thankfully, Claire prevents them from finding out about this secret world, through they unwittingly mingle with the Trolls at the party, who she and Mary Wang believe to be college students. By the end of the episode, their friendship is rekindled.
She may be romantically linked to Toby as of episode 24, when it is revealed that she is the mole mascot that Toby has become enamored with, as she was the only girl in school who wanted him to ask her out. But as he was too busy asking out the other girls.