Deimos' 'costume'... Well, it wasn't so much costume as minor body modification paired with weird clothes. But he'd invested a lot of energy and time to get his floppy ears and chubby nose right. The furry pants he'd found at a second-hand shop and they fit perfectly! Like his brother, he'd slapped a simple black domino mask on his face and entered the room, playing a flute.
A couple dozen fizzy drinks later, he was following Atropos around, drawn in by the impressive bunch of feathers she was wearing on her head, accompanying each drink she took with an attempt at a fanfare on the not-fanfare-suitable wooden flute. It was the thought that counted, right?