Do they have any siblings? What’s their names? What is their relationship with them? Has their relationship changed since they were kids to adults?
River has one sibling, a brother, Fergus. She is, after all, a Cousland. Fergus, being several years older than her is semi-parental towards her when she’s young, trying to get her to be a proper lady, but when it becomes clear around fifteen or sixteen, they adopt a more friendly, more sibling-ly relationship.
What is their favourite food?
I have to say whatever the Fereldan version of fish and brewis is called. It was probably something her mother never wanted to see again, after she stopped being the Seawolf, but something River talked her into making and developed a massive affinity for.
Do they like to take photos? What do they like to take photos of? Selfies? What do they do with their photos?
In a modern AU she’d be the type to snapchat pictures of her girlfriend studying with like, really lovey-dovey, ‘I love my GF’ captions.
Do they have a good memory? Short term or long term? Are they good with names? Or faces?
River has a great memory for faces, but not always for names. She doesn’t have the best memory for facts either. Mostly, if she remembers your name, and when you met her, it means either you’re important (ex: Fiona. Maric.) or she likes you.