I think for Cain to return you have to have very high trust levels for everybody, because essentially its the best ending.
It's actually harder to get the worst endings where Bo betrays you, Cain is lost to Amada and Rachel dies because even if you choose the wrong answers, you always seem to increase NPC's trust simply by being good at the game and getting headshots etc. The game steers you away from the worse endings and it seems you need to actually try harder to get people to dislike you.
Cain is easily the best robot character in a videgame since HK47 in KOTOR. He's f**king awesome.