It's a bit difficult to keep this topic concise, so I apologize for the novel, but companions will help with both combat and conversations. They will have their own scenes (both sfw and nsfw) and will also bring additional content to scenes you might have already encountered (nsfw). I haven't considered having them affect stat checks, but I'll definitely explore that and see how it feels.
As for the status building, I assume you mean the relationship between MC and the companion? If so, yes that will exist. It will function the same way as other relationship building (conversational, decisions, some scenes, etc). The three main ways of building relationships will be:
- Talking to them, either at their residence or directly if they're following you.
- They'll pick up on what you say to (or do with) other people
- You can invite them for fun time (if they're up for it).
I think that addresses everything you mentioned, let me know if you have any other questions!