Responses

She's definitely over 20, closer to 30 judging by:
- When Rebecca shows the file to Kiara (end of Chapter 1), Kiara says Daniela (blonde girl) is 14 years younger. The translation doesn't make it clear whether the one who is being compared to Daniela in age is Kiara herself or Rebecca. That doesn't make much of a difference anyway, since Kiara and Rebecca speak casually, which indicates they are close in age.
- Kiara says Rebecca should charm Daniela with the appeal of being an older woman.
- Rebecca is drawn with wrinkles typical of the design of characters in their 30s.
The age gap is creepy but the yuri drought is too strong to skip on this so I'll just have to fool myself into thinking they're age appropriate for each other.