Don't leave people behind, please! part 4
When Sera noticed the way Risa flirted with Ryu she wasn't worried in the slightest. She knew the instructor's intentions and was completely certain that she wouldn't try to take Ryu for herself.
What troubled her was Anne's behavior. It was clear she was more than happy after being reunited with her friends, but she stared at Ryu whenever she thought he wouldn't notice making it obvious that she was interested in him. That was a problem, at least for Sera, because she knew Anne wouldn't want to share Ryu and would try to seduce him and keep him all to herself.