Nobleflower and Quillkiller
Rita runs some sort of gossip magazine in Hogwarts. She knows everything about everyone, and has been able to confirm or deny any rumour going around the school since she started there, so when people start whispering that Alice Fortescue, the Gryffindor chaser, is secretly dating someone, Rita has to look into it.
Alice and her mystery partner have been careful, and they have somehow even managed to escape Rita in her new Animagus form. After almost a month of sleuthing, the couple finally let their guard down. Rita follows Alice to an empty classroom at midnight and sees none other than Bellatrix's sister, Narcissa Black. (At this point, Rita and Bellatrix might be dating, or Rita might only have a crush. I am a strong believer that Rita fell first and Bellatrix fell harder. I also think that they would have shared a dorm, so even if Bellatrix pretended to despise Rita, they would have had some sort of close relationship. I'll probably flesh out their dynamic in a later section).
Rita is conflicted. She knows Bellatrix will never forgive her if she releases this information about her sister, but she prides herself on always spreading the truth, no matter what. She has even shared secrets belonging to her own siblings/cousins/other family (depending on what family you think she has). She spends a full week debating what to do, retreating from Bellatrix as she does.
Bellatrix is worried that she has done something to Rita.
In the scenario where they aren't dating, she is worried that she finally pushed Rita to far, or said something too bad. Her family desensitized her from insults, and she often doesn't know when you stop, which has ruined many past friendships, but Rita has never been deterred, even when Bellatrix was actively trying to hurt her. She is too proud to attempt an apology, so instead she thinks about it constantly, losing sleep. She refuses to acknowledge the issue, instead hoping that Rita will come to her so the two of them can fight. Fighting is her natural instinct from growing up in her house, and she falls back on it when stressed or threatened.
If they are dating, Bellatrix is worried that Rita wants to break up with her, and she is angry. She hates feeling vulnerable and Rita having that power over her is unnerving.
Regardless, Bellatrix wakes up one morning to find Rita gone, and the next issue of her gossip magazine on her bed. Rita always leaves her one the day before releasing it, so she gets an inside scoop before the others. Over the years, Bellatrix has become her unofficial editor. She opens it to find out that Alice has been having secret trysts with the Gryffindor Captain, her best friend, Frank Longbottom.
Rita is the only one to ever know that she discovered the two girls that night.