Here is my RP serial killer:

1. TYPE
No type. Must be cute, but for me women are cute on a case-by-case basis. I have broad taste in women.

Should be "breakable" – but then, everyone is breakable. No buddhist monks, I guess. Another vague answer.

Lastly: no cruel, small-minded, or bigoted women. I want to like my victims, so I'll be murdering the good guys.

(There might be shitty people in this RP, but I just kill them quickly without following my M.O.)

2. M.O.
Long-term captivity. Severe torture, but usually with rest breaks afterward proportional to the severity. In some cases: limb amputations (either quad or just arms), removing teeth and nails, sewing lips shut. Mostly non-permanent stuff though.

When I'm setting up the torture, I wear some kind of face-covering outfit. Then I take off the outfit and help her through the ordeal. If she has no arms or no limbs, I help her eat and go to the bathroom. I scratch her nose when it itches, etc.

She knows I am the one torturing her, but my kindness outside the outfit seems so genuine, and she loses the will to associate her caregiver with her torturer. This deep in her suffering, she'll accept human comfort from any source, even me.

She begins to feel a confused sort of trust and even affection toward me. I become two different people, the torturer and the caregiver, and the caregiver cannot help what the torturer is doing. The caregiver is as much a captive as she is.

At the end, her caregiver goes away. She dies by slow suffocation. Drawn out for days, with an IV drip maintaining her blood pH against the buildup of carbonic acid. Alone with her faceless, silent torturer as she suffocates and the clock ticks.

I leave her body somewhere in public, naked, in a humiliating pose, usually with her legs spread and one hand on her pussy like she's masturbating. I leave her ball gagged, and her nipples pierced with heart-shaped padlocks instead of rings.

3. Trophies
Recordings. Also, I am a sculptor, and I create lifelike statues of my victims with cute, despairing expressions. Each victim watches me make her statue. While I work, I wear the face-covering torturer's outfit, and she dangles in strict bondage.

4. Name
The park bench killer? For leaving women posed lewdly on park benches.

5. Further Info
I do not always kill. I have collected a few women to keep alive, all of them limbless, in a lower basement under the main torture basement. The characters in your RP might break into my house and find one of their long lost friends still alive!

She'll be totally helpless, and probably pretty traumatized, so naturally someone will need to care of her once she's out of the hospital. This could be a very cute way to pair up two characters, or even have the whole group care for her together.