"Eri is difficult to write since we don't know anything about her!" "She doesn't have a real personality, we don't see her at all..." Guys guys guys... You are SO WRONG it's not even funny. We know many things about Eri, actually! It's easy to build her personality from that. You just need your brain and a little bit of imagination! But since I'm a little bit too obsessed with twewy, I made a list of all canon facts we know or can easily guess about her. Feel free to add to it!
- First, we know that Eri is super popular. She has many friends, we even see her hanging out with Mina (in game) or an unnamed friend (in the anime). For both times, neither of them know who Shiki is, despite her being Eri's best friend. I think we can guess from this that Eri has many friends, but none of them know her like Shiki does—in other words, she doesn't have many true connections with others.
- Then we know that Shiki's self-esteem issues aren't really new. For someone who's her best friend, Eri has been really blind about that (maybe blinded by how herself put Shiki on a pedestal and admired her?) and didn't do anything about it. And when she finally did something, it was very badly said ("you're not meant to be a designer" geez Eri, are you sure that's what needs to be said to comfort Shiki, struggling on a design?).
- Finally, we know that Eri does have a sharp eye, but only for fashion and trends. It's said she always notices mistakes on Shiki's designs and always points them out. She's also said to be very smart and successful at school, and of course, very pretty.
- Other less important info about her are that she likes Def Märch and that her favourite clothing brand is D+B.
Remember that this is a non-exhaustive list of all we know about Eri from canon! I may have forgotten lots of stuff. Hope this helps! ^^