It's the rival receptionist. The reason they die and her affection drops
The Herb Quest sent him straight to a dangerous clilff he'd have fallen to his death if Jibril didn't carry him. Fafnir-chan was setup to send people to die, because she was scouted to the guild directly by the Headmaster. We clearly see the rival receptionist has jealousy and anger at Fafnir, so it would make sense if she is altering her papers to indirectly kill people that interact with Fafnir. Fafnir develops depression and a bad reputation that way.
As for the affection we clearly see Fafnir is bullied the moment the MC is away. The receptionist hammers in her head that she is jinxed and will kill him, so she gets rid of any feeling to not be attached. Her affection rises super fast at the possibility of an adventurer that survives and proves she isn't cursed, but falls equally as fast the moment the Receptionist tells her it's her fault and she will kill him like others. And she does so with full confidence because she has been falsifying whatever information Fafnir will give the adventurers and therefore make her send them to death.