I had the idea since the beginning that you could create some “pre-game” storylines. As such they would not influence the main story, besides giving the player access to some new (better) team mates at startup. the idea is that you play a small set of solo missions with the character in question, and it ends with that person joining/finding the Phoneix Point base.
Examples:
ADAM-7. One of the first PURE created by the PURE themselves (ADAMs and EVEs). He does not, however, buy into the whole anti-human thought, and decides to escape and find someone to help him oppose said mindset (this would give you the oppotunity to have a unique PURE on your team).