He doesn't think she is joking or being sarcastic, he thinks she is talking about a kitten's body not a human body. When she said she was not talking about the cat's body he was distracted, so he still doesn't know that she is hiding a human body in her house.
I would also say they are similar but he is in an advantageous position as he is more socially aware of his surroundings.
My theory is that either he had hurt animals in past or he had been friends with someone like that. I incline more to the second one because in some pages the lines he says seem to indicate that he thinks that she is mentally sick.
To be true to me he seems like a person who doesn't care what is happening as long as it doesn't involve him.
She's not a sociopath or a narcissist, just someone who craves attention and feels insecure emotionally. She likes to be seen as “weird,” “crazy,” or even jokingly called a “psychopath.” She gets a kick out of people's strong reactions, especially when they're fascinated, intimidated, or amused by her. This “crazy” image becomes part of how she sees herself, so she might even exaggerate her behaviour to keep that identity and keep the attention coming.
But underneath all that, she's not really emotionally tough or detached. She's actually pretty insecure and relies on others for validation. When people question her or say she's not as crazy or unique as she acts, she gets upset because it threatens the image she’s created for herself.
Her friend group knows it’s mostly a performance, but they still go along with it because it’s entertaining. They call her cool and feed into it, but when it doesn’t benefit them, they back off or act distant.
The long hair girl in the group is manipulative. She acts friendly, but mostly just pushes the purple one further into the “crazy” act for fun.
The orange-haired guy is into girls who acts psychopathic. He reacts strongly when she acts that way and she likes it. She doesn’t actually like him, but she likes the attention and reaction she gets from him.
Then there’s the green-haired older guy she likes. She knows he’s probably just using her but she still wants his attention. I think that why she started acting like that because he was the first one to call her a physcopath and they dated to after that.
But later on It became less about liking more about emotional validation. When his attention changes, it affects her.
I also think she my do something drastic or might became the perfect scapegoat for our MC.

i think its really interesting how a lot of the main characters in this story follow textbook of the dark triad concept, like narcissism (the bully girl), psychopathy (the mc), and machiavellianism (the student president guy)
the student machiavellianist president guy is the most interesting character because hes so like fake charming and obviously manipulative. the main thing about machiavellianism is the "means to an end" and he just establishes a relationship w/ the mc for a better looking record. hes cunning because he knows his position well and exactly how his effort for him being a star student to everyone can be used. he doesnt genuinely empathize with our mc, he just found a way to look better on student records.
i think its kinda ironic that he didnt understand how he was similar to our mc, but they both share the low empathy, the strict rule system, and he understands the way she kinda thinks in a wayy.
he, and like many others, have dehumanized the mc because of her behavior, but hes kinda just like her fr LMFAO
i did think it was lowk crazy how he didnt care about the mc literally admitting to the body in her fridge but thinking about it it makes sense why his character doesnt gaf.
i have multiple theorys, like that he thinks shes joking/sarcastic, but he knows better than that bcuz he understands her well. maybe he does understand that shes not lying but doesnt want to deal with the actual nitty gritty of her, or he doesnt care to know (which i feel like he could totally use that against her so idk), or maybe hes even dealt with bodies in his personal life before