Responses
I can't see the "harlequin" in Kubo(if someone can, then feel free to explain hehe), but I can in Eichii (the model). Cuz it's ironic how all clothes look perfect on him, but at the same time that fact makes him become "colourless"? Like, instead of looking perfect in all kinds of clothes, he cant look perfect in just one kind of clothing. This makes sense of when the woman mentions about how plain he can be.
Harlequins are traditionally mischievous "devil-like" servants who wear masks and colorful clothes and are always trying to compete with their "masters". It's interesting that the woman mentions how plain the model can be - the designer said his clothes were made for him, so we might see how the right clothes will turn the model into a colorful character.
My guess is that, in a way, both of them are harlequins. The word "Goodbye" is just stating that there will be a moment when that Harlequin facade will be left behind, and transform into something else (like in Goodbye Lilac and Sayonara Game).