For mostly autistic and variably ND people, they have what's called a 'safe person'. For some lower needs nd folks, what this person provides is a sense of safety to unmask. Autism advocates on other socials have defined this as a person who has "seen all sides of them and has not judged them for it". The 'safe person' gives the autistic person space to feel the true extent of their feelings, good and bad, outside of external pressure from neurotypical expectations. Regardless of the relationship otherwise, it's the context of that alone that defines what a 'safe person' does for someone autistic.
Revenant is Loba's 'safe person'. I know it sounds counterintuitive that Rev be "safe" in any sense of the word, but he is the only person she has found herself around that has never questioned her emotional reactions. Maybe he's made them worse, but he has never told her to cut it out like others have (cough Kairi cough). Revenant has never pressured Loba to mask, and in fact, encourages her not to.
I posit that this is the reason she was so reluctant to kill him. Yes, she wanted him to suffer, but she said something really key during Life for a Life: "Better than letting you go". Not 'better than letting you die'.
More than anything, Loba wanted to hold onto the freedom to unmask around Revenant and take out her frustrations and trauma on him. When presented with the option of not having that anymore, she had a meltdown.
Note: Anita has tried to be Loba's safe person, and prolly very often times is... but not to the extent that Revenant has proven to be. Anita has also had her own issues to work through separate from Loba, including being around her brother again (being that I also think she's on the spectrum sincerely). I believe that one day she can in fact be Loba's safe person again in full, but someday.