They're not a couple. They are friends. I live with another dude. We pay 400 dollars each to rent an apartment close to our university. And I assure you we're not a couple.

Wendy and Chelia have always been stated to be friends. Also, one of the last chapters disclosed and confirmed Wendy has never done anything sexual with another human being (chapter 508). It's a confirmation, Wendy and Chelia are just friends and did nothing during that year.

In the future the author may change that if he wants but so far they are merely very good friends.