Alright, guys, here's the deal: We've found out what caused the issue and fixed it for the next update. It turns out that when Six teleports away after the player speaks with him at the Institute, in some cases his characters does not save properly, preventing him from showing up at the start of DC; in some of his other appearances he is directly spawned during a cutscene, so the problem does not manifest there. So the current workaround is not to save your game once you've spoken to him at the Institute (if you want to speak with him, that is). Go through his dialog and reload. Your choices there will not influence anything major but will only change some of his responses once you meet a bit later, so you won't be losing much.
Now, there's also a bit of bad news: We've done some changes to the game code and it turns out (as Megaost later informed me) that the fixed saves do not work in the release version. It would require too much work to make the saves compatible, so everyone who has been affected will have to wait for the update. However, it is entirely possible to finish DC without Six's help; the quests he gives you should also be given later once you come near your objectives. I recommend you find a Youtube video, read what he has to say and continue your game. Or wait for the update, but we don't know yet when it will be released.
Sorry for the inconvenience and the long post. tl;dr: Not available.
