I just had a random thought. Since all hidden characters have the same requirement to unlock them Spoiler (meeting them in subspace emissary and getting them to join you) and the events are scripted, there's really no way for that NOT to happen...unless of course you die and then retry. So in THEORY you can unlock everyone within one play through of SSE. It sounds like the faster route to a full roster as opposed to countless VS matches. Although I'm sure the latter is the more enjoyable way to go about it.