The author has a list of romancable characters on their tumblr page and Aeran is not on it. The reason is spoiler, apparently :)
Not all companion characters are LI but there are characters that are not part of the main gang yet can be romanced. There also will be characters with whom you can just flirt with or have one night stands with, without them being really LIs. So yeah, just because you can sleep with someone, they might not be romancable.