Sera said to ran in the post-credits scene (M18-Sniper From Another Dimension) that only Shinichi could snipe her heart and also commented,”For Now Anyway…“Does that means she is going to snatch away Shinichi from Ran???
It’s clear Sera’s in love with Conan.
She appeared for the first time when Sonoko told Ran “One day a rival might appear you know?“,
she told Conan, while talking about herself “Do you have a love interest Conan-kun? For example, a girl you met a long time ago, and couldn’t forget…“.
And also her ambiguous “For now, anyway” in movie 18.
This theory holds more water than it first appears to. It could explain Sera’s obvious attempts to get Ran to figure out Conan’s identity. Ever since Sera appeared, and especially in the presence of Ran, she’d been putting Conan in awkward positions, usually by either pointing out something odd that he’s doing or bringing to Ran’s attention Conan’s strange behavior. It’s possible she wants Ran to know who Conan figuring that she’d be so upset with him she’d reject him. Besides, when Sera was first introduced, she was implied by the manga chapter and the dialogue to be a love rival. Her overprotectiveness of Conan and constant blushing around him lends credence to the idea that she’s in love with him.
Also, it’s possible that Sera feel guilty about what she’s trying to do, hence the “Maybe I’m a bad kid after all” in the Yuusaku cold case (ep 690-691) after she sees Ran giving Conan a bandage. Besides, that came at the end of a case that Sera spent trying to get Ran to figure out Conan’s identity.
She can’t be a relative too. Ran had Eisuke, maybe it’s Shinichi’s turn to have a new character who loves him. Maybe Masumi’s going to trigger Ran’s confession to Shinichi just like in the London arc.