Which of my birds do you think this is?

My head hurts fr reading your Q (ha). A pet bird might become more friendly towards it's owner if it loses it's cage mate. Really depends on the bird. The one who died would likely have been eating less and so would be skinnier than you would have been able to see by just looking at it in the cage due to the feather cover. I'm thinking it's Millie who's left because a birds personality likely wouldn't change that drastically in such a short time. But....I'm just guessing. What's really important is that the one who's left is being outgoing & you should encourage that. Whether or not you're "just crazy" I cannot speak to (lol). Just keep enjoying whomever is left---it's either one or the other (sorry, couldn't resist) <3