What is the part without the radical called?
On wiki, this part is defined as "phonetic". I am not familiar with these jargons in English, but I thinks this one is acceptable.

In your case,"艮"(gèn or gěn) is really a character.

Edit: Add Chinese names for this part

We call these characters “形声字”(in modern Chinese, many characters can be classified into this type). 形 means "form", referring to the radical, 声 means "sound", so I think "phonetic" is acceptable to call that part. 

In Chinese, we call the "radical" 形旁 and the "phonetic" 声旁. Here, 旁(pang2) means a part.