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This one's a complicated tangle, because there are two sides to it: what the reader knows and what her friends know. At this point Sandra's effectively a Mysterious Stranger and Wild Card with a familiar face to her friends. (Possible YKTTW, "Mysterious Former Friend"?)
At the moment it could end up a lot of different places- Sandra's probably realizing exactly how far she fell and that she will be The Atoner if she comes back to the group: At best her actions could be Forgiven, but Not Forgotten and at worst, she's facing a potential Heel–Face Door-Slam.
Regardless of the rest, Crystal is her Morality Chain.
Edit: reread that second paragraph...yikes. That's what I get for trying to type and talk at the same time; made some tweaks so word bundle now make are sense now.
Edited by Scorpion451Thank you very much for your answer! I think I will wait a little before adding some new tropes. I'll still add Morality Chain (completly forgot about that one, which wasn't listed yet on the work page).
Kinda unrelated to your request but Crystal and Sandra seem to go through a "I Know You're in There Somewhere" Fight.

Hello,
What is the trope for when a (good) character is expecting another to finally make an Heel–Face Turn, and decides to give up on him/her because said another character doesn't give them a straight answer?
Context: in the webcomic Zebra Girl, Crystal, the best friend of Sandra, the titular "Zebra Girl" who is a human turned into a demon, discovers that she is back after 5 years of banishment. Sandra was banished because she went full-demon mode (she was turned into a demon at the beginning of the strip, tried to keep her humanity, and at one point decided that if she couldn't be a normal human, she would be a normal demon), terrorized her hometown and attacked her (former) friends (going as far as torturing one of them daily - said character, who was a badass, is now suffering from severe PTSD). Crystal still hopes that maybe Sandra came back to her senses, because if she didn't, they would have no choice but to end her permanently (her current boyfriend, a werewolf, is out for her blood). In this strip
, she goes to her hometown alone (ditching her friends to do so), calling out for Sandra to show herself and give her a sign that she has changed (Sandra has, but as far as Crystal's group is concerned she may still be dangerous and vengeful). Crystal kept the hope that Sandra would be back, but she is afraid that the best friend she knew is definitly gone, and she is pleading her to give her a sign, any sign, which would prove that she is no longer batshit insane and that they don't need to kill her (and that she won't hurt them either). Please also note that it is heavily implied that Crystal and Sandra's relationship goes beyond the frienship spectrum, with Sandra being in denial about her feelings towards Crystal (having realized just recently that maybe Crystal is more to her than just a friend - even when she went berserk, she avoided being pointlessly cruel towards her, and admitted that she misses her), and Crystal who may or may not have romantic feelings for Sandra.
Edited by NonoRobot