We’re not sure. You can never get a straight answer out of him.
The AESIR sent him as a knowledgeable and valuable hostage during their truce with the VANIR, but it soon became clear that he was cribbing his speeches from MIMIR. When MIMIR wasn’t around, HONIR became a gibbering imbecile. The VANIR took their revenge by cutting MIMIR’s head off and sending HONIR back with it.
Also known as the Silent One, sometimes he might not be there at all as he has very long legs and can vanish from sight very rapidly.