Badass: Cirno. Her fight against Star Sapphire is proof enough of that.
Beware the Nice Ones: Aside from those already driven crazy, Kogasa goes from cute, assuming youkai, to saving herself from Sanae's Cold-Blooded Torture via umbrella impalement, all while crucified. It's clearly desperate self-defense rather than Unstoppable Rage, but the way she gives her trademark tongue expression and chants 'Urameshiya' beforehand is....
To put this in perspective, the first real fight in the series is when Meiling prevents Koishi from entering the Scarlet Devil Mansion. She gets the entire front of her neck torn out so that the spine is visible.
Sanae captures and tortures Kogasa, before getting killed herself.
Mokou destroys the human village, and kills everyone in it.
Yuuka does...something unspecified to Medicine Melancholy.
Star Sapphire literally pulps the other two fairies with a chainsaw.
Tewi breaks Reisen's legs with a mochi-pounding mallet.
Satori decapitates Rin with her bare hands...while being carried to safety.
Reisen II kills Yukari by detonating a bomb implanted inside Ran's stomach.
Blue and Orange Morality: Calling Koishi outright evil is a stretch, but she's definitely not playing by anyone else's sense of morality.
Femme Fatalons: Koishi has hands with very long, claw-like fingers. Their exact length tends to change, though.
Fingore: When Koishi gets locked up in a room, she tries to literally claw at the door until all her nails are BARELY attached to her fingers. Then when the story comes back to her again, we find that all her nails have fallen off. Squicky indeed.
Perhaps Tewi didn't actually crush the real Reisen's legs?
Exactly how canon that part was seems to be debatable, because at the same time, Yuuka was seen seriously injuring Medicine. Yuuka later appears in an image along with the heroes, as someone trying to keep the peace. It could be though that Medicine was just as evil as everyone else.
Poor Communication Kills: Remilia, toward Flan. Remilia was not there to save Meiling or Flan from Koishi's rampage, because she was at Reimu's discussing Flan's improvement and showing the parasol she had planned to give Flan when she returned to the mansion. As a result, Flan believes Remi only sees her as a burden, which is why she wasn't saved.
Patchouli: You really don't know how to talk to her, do you?
Power Copying: Koakuma's power is apparently the ability to steal powers from the dead, implied by her use of a 'miracle' after Sanae's death. Confirmed after using Sunny's, Luna's, Mystia's, Youmu's, and Cirno's own powers to fight Reimu.