Plumbird's hidden trait

So I've finished the Plumbird's quest-line and I wasn't satisfied with the answers I got.

You can't normally add Plumbird to your family, so I used clone machine. Resulting clone was just a normal sim.

So then I added Plumbird to the family "forcibly" (shift+add to family). She immediately lost her "costume", but what's more important - she still kept the special treat called "Plumbird in the past" or something similar.

Anyone knows what this trait actually does?