WARNING: UNMARKED SPOILERS

The Tails Doll is a misunderstood soul locked up in the Companion Cube by GLaDOS because she figured he was dangerous (Like The Internet thinks he is). She said it would never threaten to stab Chell as Self-Assurance.

Between both games, the Tails Doll broke out of the Cube and rigged GLaDOS to come back to life if anyone other then Chell tried to escape. Namely, Wheatley.

[Spoiler: At the end of Portal 2, GLaDOS found him, trapped him in another Companion Cube and scorched it to make it look like the old one, and gave it to Chell so it would kill her and whatever was left on Earth.

The Tails Doll breaks out of the Companion Cube and goes back down, has a massive battle with the Turret Opera, and is owned by GLaDOS...

Cue the Co-Op mode...

edited 5th Jan '12 10:17:42 AM by GammaWALLE

So that puts the whole of the sonic universe in with the half life/portal universe? That, or the tails doll is a dimension hopper. I like both of these ideas, honestly.