From what I get, what Riko loved about Sae was that Sae hated her. She was different from those who unconditionally loved her and it was evident by how she never took her picture(because she wasn't fawning over her and saw her as regular friend). Sae was more than just someone mindlessly attracted to Riko and that's what made her special. What Sae liked about Riko was that Riko was always looking at her(something she didn't realize til this chapter considering she says that she found out why she loved her). She didn't care who,she just wanted to feel noticed by someone because it's clear she lacks attention due to living in Riko's shadow. When Koike started showing interest in her was when she realized that she never liked Riko and after he stopped,she realized that Riko was all she had. Riko made her stand out and feel noticed. Although she did want to be noticed more,I think she was fine if at least one person looked her. Same thing with Koike,when Koike started liking her she didn't care at all that everyone else disliked her. When Sae took the picture of Riko,she was no longer unique in her eyes and thus Riko moved on from interest in Sae, leaving Sae to be just another in the background. Would like to note that more than like or love,it's more like a heavy and twisted form of dependence/interest.