Seth saw Seren was a bit confused, but since he wanted to clarify everything for her, he explicitly told her about his relationship with Rina and Arwen.
Serene didn't feel jealous after she discovered that Rina was Seth's girlfriend as well. If there was something she felt after hearing Seth's words of explanation, was gratitude and curiosity for Rina and her mother.
Actually, for demons and devils, having more than a single partner was a very normal occurrence and even her own mother had two husbands at the same time when she was the empress. For the demonic race, what mattered the most was power; the strongest ones, would rule over the others in the way they wanted and that was one of the reasons Serene didn't intend to fight back against Velgor's marriage alliance with the Lion's Pride Golden Sanctuary. She would rather suicide instead.
