I did my best to write Ein's wiki page, but got quite confused by her clothing.
To me it looks like a jacket, but others have pointed out that it could also be a dress, and I now also share that opinion.
"ein_(blue_archive) solo" yields 118 posts.
"ein_(blue_archive) solo dress" yields 33 examples.
"ein_(blue_archive) solo jacket" yields 10 examples.
I'd appreciate a reply in here or an edit to the wiki.