Have to go with hitomi, she's a black belt, it's confirmed that none of the other students at the dojo could beat her, is the runner up consistently in the DOA tournaments and she evenly bests Ayane in their fight which says alot.
Kokoro I feel wouldn't be able to keep up, she has some potential but her style around DOA4 was unrefined, and in the DOA3 events when she encounters Ayane, she's defeated and had to run away to escape her. On top of that, she never does that well in the DOA tourneys she enters, and it's clear she lacks the emotional strength to process her faults since she'll call for her mother when she's defeated in doa4, and she's quick to get angry at Lisa for simply bumping into her, but she was super slow processing the explosion that took place in Tokyo when Helena made that DOA5 announcement, and when Jann tossed that nunchaku at her, she literally reacted in slow motion and had the audacity to ask what he did it for like she didn't have time to react. She's too emotionally driven and physically inept to be able to contest with Hitomi who is very in the moment and can read the room.
As far as interaction, I can see them befriending each other although Hitomi's more out there and energetic personality would definitely clash with Kokoro who's more reserved and quiet, but I noted both of them are quick to anger when they feel they witnessed or suffered injustice. I don't see them fighting for any reason besides in tourneys but even then Hitomi would easily best Kokoro.
