Outside the Hestia familia she's the only other one who realizes that the monsters are sentient and decides to not fight them. This is also why Ais in particular is also turned into a mushroom. When seeing the Xenos wearing armor and showing intelligence Finn says, 'A monster will always be a monster', Bell tries to defend Wiene by claiming her as his prey, and everyone turns against him. Now with the context of season 3 this becomes much clearer, she was dreaming of the events of season 3. She tells everyone, no one believes her, the end. The we cut to Cassandra waking up, everything we saw was her dream. Finn shows up and says something along the lines of, 'A mushroom will always be a mushroom', Bell says 'These are my game/prey' which makes no sense in the context, and then everyone turns against Bell. Well Bell realizes that the mushrooms are alive and his friends he instantly decides to protect them. Basically everyone gets turned into mushrooms to make a long story short. Near the end though, things start to get weird. The majority of the OVA is just standard beach episode fan service, you either like it or you don't. Her dreams are also never literal, the meaning is there but it's often metaphorical, for example Bell showing up as a white rabbit. She has prophetic dreams that always come true, but nobody around her believes in them. Let's start with some context, in season 2 Cassandra is introduced. If you check MAL for this OVA you'll probably see a flood of 1 star reviews claiming that everyone involved in making this OVA must have been on thirteen kinds of drugs, and to some degree that is understandable. So obviously spoilers for this OVA but also spoilers for season III of Danmachi.

Danmachi ova