Westcott is her boss in which she has a superior-subordinate relationship with. She does sometimes disagree with him however, sometimes questioning some of his plans, such as why he didn't let her kill Shido when she had the chance to.
When Westcott ordered her to observe Shido, she originally thought that he was an ordinary human. However, his ability to summon an angel peaked her interest, causing her to wonder what he was. After failing her mission, she asked Westcott if it was possible for a human to wield the power of a Spirit. Over their many meetings, Ellen has grown to greatly despise Shido. However, in Itsuka Disaster, a not himself Shido started to flirt with Ellen. Using his spirit powers he easily stopped Ellen's attempts to keep him off her back. At the end of that meeting, Ellen swore revenge for the humiliation that Shido brought to her, eagerly agreeing on Westcott's request to capture Shido afterwards.
While its generally unknown what Ellen's relationship with Mana was like when she was still in the DEM, she has grown to despise Mana as much as Shido, stating both siblings really know how to make her unhappy.
When the two of them fought each other (in Volume 12), Ellen stated that she should have killed her back then, implying Ellen was the one who separated Mana from her family.
Tohka is a Spirit and therefore Westcott wants to capture her and achieve his "goal". He often sends Ellen to try and capture her.
Ellen greatly underestimates her strength after she defeated Tohka twice in her limited release Astral Dress. This would backfire on her while fighting Tohka's Inverse Form, forcing her to retreat after stating that she wasn't able to fight at full strength because of a wound she had gotten from Origami.
30 Years ago, Elliot helped Westcott and her bring the First Spirit into the world. According to Westcott, she would be overjoyed if he came back to DEM. However, once it became clear that was impossible, Ellen swore that she will personally hunt him down and kill him for breaking their oath.
Karen is Ellen's younger sister who works for Ratatoskr's head, Elliot Woodman. For reasons still unknown, they are not in good terms with each other and are considered enemies due to the conflict between Ratatoskr and DEM.