Quote from Kaitentsuki
I always believed it was because of the deal she made with Headon, or maybe it was due to her wanting sympathy and disliked Baam for having what she did not have, she used Baam to get into the tower meaning she was an irregular since the gate actually opened for Baam and she even said she was afraid of the night at some point referencing Baam whose name is read the same as night, so it could be both jealousy and fear. Baam is technically an irregular and a regular, the gate opened for him but he did not go through but afterwards he opened it himself meaning he has the same aptitude as and irregular.
Yup! Keypoints right there. Remember that flashback chapter when Rachel was denied further access into the tower because she's not 'the one?' If anything, she's the irregular while Baam was the designated regular who didn't manage to get through. Rachel then became jealous and betrayed him because she's afraid it would get out that she wasn't supposed to be a regular, OR maybe she's afraid that Baam would take whatever she's looking for away from her. I don't think she wanted to share like what Baam proposed, she just wants things for herself.
Detective Horror Series