A quick daily log of how you are doing. Your companion sees it, remembers it, and brings it up later — which is the difference between a diary and a relationship.
You record how you feel in a couple of taps, optionally with a note. The entry is stored with your account and your companion uses it as context: a rough Monday is something she can ask about on Tuesday.
Over time the log becomes a picture of your own patterns — which is useful on its own, and a good reason to keep the streak going.