This includes a diary. Start keeping a diary of events. Dates this person called. What was said. What you saw. What you found out. Someone called you about this person. You saw them with your loved one at the mall. She had a black-eye at dinner. However, small it may seem to you - write it down in a diary and document with evidence if you can. I've seen whole cases won on a diary. Otherwise, you would be shocked what you can't remember a year down the road when you're stressed out and worried. Remember, people don't care what you feel or think. This diary is for EVENTS or things that happened that can be used as evidence later down the road. Things like "took her to the hospital to get a cast on her arm" or "she showed up at the house with two black eyes". Those are the kinds of things to record in your diary.
Now this is going to sound harsh - but you might want to make sure there's a DNA sample somewhere. I keep hearing about bodies being found all the time and they have no way of identifying them or matching them with a missing persons report because no one has any DNA of the missing person. If you can collect hair that has a root on it, or maybe this sounds disgusting but an old tampon you pulled out of the trash would have blood on it, but you need to think about ways to get blood, saliva, semen, or a hair with a root on it for DNA evidence to store some place safe. I have heard over and over again how these girls just go missing in one day and no one has any DNA evidence, fingerprints, dental records, etc. Then when they find these bodies - there's just no way to match them up. So think evidence now while you have this person in your life if you're worried something might happen to them.