That's alright. I don't share much of it and it's gotten pretty complicated.
As much as he would love it to be the case, Aezar is not nor can ever be an item with Elethia. They are very good friends and spend a lot of time nerding over alchemy together, and he developed an emotional crush that turned into love (that may or may not be reciprocated).
Unfortunately Elethia has her own hangups about dating mer, even if she's currently unaware this one also happens to be undead. So for both their sakes she is leaving, traveling abroad for research for her book.
Aezar and Cassica have been frenemies for much longer. They worked together on the odd contract for the Dark Brotherhood but otherwise theirs was mostly an abusive relationship, full of inappropriate comments and unsolicited advances from his end which were usually met with a punch to the face (or groin) from hers.
Aezar was literally the last person Cass would have considered, but her near-death experience shook them both up. Now things are complicated and it turns out Cassica may have more feelings than she's letting on.
At the end of the day Cass is the only person Aezar can completely be himself around and vice versa. They've known each other for so long their quirks have become almost endearing, and despite what happens they've got each other's backs.
It's complicated. At the moment Aezar is still dealing with too many feelings and fresh heartbreak to really appreciate Cass, but she might literally be the only girl for him.