A crush, in my understanding, is often one-sided feeling. The other party may not even know it (many high school girls have a crush on movie stars). So in Friends season 1 episode 1 when Ross told Rachel "You know, you probably didn't know this, but back in high school, I had a, um, major crush on you" and Rachel said "I knew", Ross was quite surprised. "You did! Oh.... I always figured you just thought I was Monica's geeky older brother." (To which Rachel replied, "I did", eliciting laughter from the audience.)
What prompted my reply in #4 was last year's phenomenal hit Your Name (君の名は。). Browsing through Taki's phone, Mitsuha found two photos of Okudera-senpai: 「あっ、あの人や！片想い…かな？」 (then she went on to arrange dates for them lol). I watched a pirated version with English subtitles. The English goes "Oh! That's her! A crush, maybe?"