She just "up and exposits about it deadpan" because she emotionally distanced herself from the events using the same powers she was shown to have: she wandered Limbo in spirit form to escape the abuse as best as she could. People
do try to dissociate themselves from trauma even without psychic powers, if you knew anything about that. There's also the possibility that they were so affected by the event that they speak in monotone/deadpan because of the effort needed to distance themselves. Or, y'know, Kat just sounds like that because she's always sounded like that.
Contrary to what you might have seen in media or whatever, victims of rape aren't all immediately rendered into emotionally frail buckets of tears that can't go anywhere near their trauma without exploding with emotions, nor do they all resort to hypermasculine behavior and become badass killing machines with guns and swords like their rape was some hidden on-switch for being cool that gave them experience points to level up with.
You know what some victims do? They
get over it. Shocking, right? They don't let their rape define who they are. They find constructive outlets so that they don't focus on the traumatic incident. They can even go for months at a time without thinking about it. They live their lives, have friends, don't let the event hover over them like a storm cloud coloring every interaction they'll ever have with anyone ever, and leave the past in the past.
That happens. That's exactly what Kat does. She turns to honing her powers as a medium. She has a trusting relationship with both Vergil and Dante and doesn't think of them as "potential rapists". She has friends in the Order aside from those two. She can
talk about her trauma, even with initial reservations ("it's in the past," etc.) instead of bottling everything up.
Turns out, the idea of all women being one unwanted penis away from getting their entire life wrecked is bullshit, and that it is possible for a woman to have a healthy, happy life as an individual
in spite of the assault and not because of it, and not be considered some weak little thing nor does her answer to everything turn into "shoot it in the face". Who knew?
@randomlulz: The "Devil May Cry" title drop was originally in the first game's booklet, with Enzo Ferino's testimony in regards to Dante. "I tell ya, he glares at a guy, even the devil may cry!"-- because a guy that makes even inhuman monsters start bawling just because of his glare must be A) incredibly ruthless to them to garner that response, and B) a badass.
But nobody reads booklets anymore.