As speculation I would say devils have rules.
Diabel hints he has another “teacher” and asks the protagonist if she’s jealous. She says no, because she’d only be jealous if there was a romantic connection with him. Instead, she’s angry that someone else can control her demon. They both declare that the other belongs to them. Iana has a contract with Diabel, meaning he must obey her until she gets her revenge. Once the contract is fulfilled, Iana will belong to Diabel.
The scene is very tense and almost sexual, with suggestive glances exchanged. Iana hasn’t forgotten the Diabel from her past life. Diabel, even though he doesn’t show it, feels uneasy about Iana getting close to Erez for her revenge. He’s neither for nor against it.