She does call herself Haru though and I actually prefer to use that instead of Kudou/Duu when referring to her... you know, as long as not in the same sentence with Harunan
Haru =/= Harunan or even Haruna. I've never heard Haruna refer to herself as Haru so when someone goes "Haru" I just assume it's Haruka since she's the only one who does.
Indeed, Harunan it's her official nick. And actually Chokubo isn't a known nickname or anything like that (I don't think japanese people even know that nick), she doesn't even have the chocolate color anymore, so it doesn't really apply anymore. Tbh, it may only cause confusion to other fans...
"Chokubo" is based on the chocobo from Final Fantasy…
Y'know, because she's tall (relatively) and thin and chirps like a bird. I don't think the chocolate color had much to do with it. In fact, it's even more appropriate now that her color is yellow.