New cat attacking resident cat

TLDR: new cat (1yr old female) sporadically attacking resident cat (3yr old female). RC seems scared and doesn’t fight back. The rest of the time they seem to get along fine. How to proceed?

Hi! Earlier this week my fiance and i adopted a second cat, 1yr old female from a cat cafe/rescue near us. The new cat (NC) was curious/loving at the cat cafe and reported as getting along with the other cats. She had previously lived with a family but was owner surrendered because their dogs would chase her.

Our resident cat (RC) is a 3yr old female. She’s a loving/playful cat but is also happy to sit on the couch and snuggle. She’s a bit of a scaredy cat but generally confident. We thought she had a good amount of energy and we travel a lot. RC always seemed really lonely and bored while we were gone so we thought it would be good to get a second cat.

When we brought NC home, we set her up in our spare room with everything she’d need. NC hid in the closet for a few hours but by late afternoon was screaming at the door nonstop. RC clearly could tell something was up and kept hanging out by the door but wasn’t hissing, just trying to poke her head underneath. The first night we let them see each other in the main area of the apartment. NC spent most of the time running around and RC tried to approach to sniff her but NC wouldn’t really be still long enough for her to get a good smell. They hung out near each other and RC mainly observed. After like 10-15 mins we gave them each a churu and the ate next to each other. RC lightly hissed one time when NC got too close to her churu but otherwise seemed fine. We then put NC back in the spare room for the night and RC slept in the bedroom with us, but kept leaving to go sit by the door of NC.

The next morning, we let them have another 10-15 mins together and it was much of the same things- NC running around and RC observing and trying to sniff her. RC did a half ass attempt at swatting NC when she was up in her face but it didn’t go anywhere. Left for work and put NC back in her separate room. Came home at lunch at let NC out and the two cats hung out all afternoon with no fighting. RC swatted at NC once or twice but it was not hard and she didn’t really follow through, and then they’d go back to chilling. Sometimes they’d do their own thing and just sit in separate areas, sometimes they’d follow each other around. That evening we separated them for dinner but they spent the rest of the night together and even played with the same pompom balls.

For sleeping, we decided to trial letting NC out of the office and RC and NC were playing together quietly when we went to sleep. RC joined us in bed shortly after and NC also slept at the other side of the bed until 4am. NC got up and was meowing like crazy and running across my pillow so around 5am i put her back in the office where she chilled until breakfast. They had a little bit of time together this morning which seemed fine and then NC went back to the spare room until we got home from work. The evening was mainly going fine with them sniffing each other, RC licking NC a bit to groom her. They each ate dinner in their own rooms (automatic feeders) but at the same time. Late evening there started being a little hissing and i thought it was RC hissing at NC but it always stopped super quick and wasn’t intense so i thought it was just them establishing boundaries. I caught it happening and it was NC hissing at RC who was cornered in the car tree. I separated them for 15 mins for cool down before letting NC out again (maybe this was a mistake 😭).

When i let NC out they seemed okay for a few mins. I scooped their litter boxes which they were both interested in watching and then they played together with a worm on string tail. RC seemed a bit wary of NC and would back away if NC got too close while running around but otherwise seemed okay- was playing and letting me pet her/coming up to me. All of the sudden NC started really attacking RC, like swatting hissing and chasing her up a chair. I clapped to get their attention to try and break it up but it didn’t last for more than a few seconds before NC started again. She sounded really angry and I was worried about RC so i threw a blanket over NC. Under the blanket she was hissing/yowling. When NC wiggled out of the blanket i picked her up (she’s only 6 pounds) and put her in the office. They’re separated now but I’m not sure what to do moving forward. They seemed to be getting along so well, they even napped on the same blanket, one on each side of my legs this afternoon but now it seems like NC is attacking RC and RC is too shy/timid to fight back.