Well, I mean, if you are a gamedev, just follow the harem anime stereotypes.There needs to be at least some story to it, otherwise u could also just jerk off to images on rule34 imo. Character relations are important, this can be achieved not by a wall of text but key moments that exaggerate that (Boy Hero Ken is a good example, some routes of scars of summer too). Of course, that is just my opinion at the end of the day everyone has their own, but I feel like NTR just really starts to hit when character relations are established, otherwise it feels just like normal porn.
No need to build overly intricate personality, just a couple of activities, a favourite place, job, and a previous connection to MC is enough.