Voiceovers main appearance is a hand, but it may not be his true form. In the episode 5 onwards intro he appears as a hand with eyes. Whether this is his true form or not is unknown.
In the first intro, he appears as a hand with a thumbs up.
Voiceover plays a main part in the series, commenting on everything, and frequently using his hands to do things in the episodes.
Voiceover first started interacting with the episodes in Episode 2, where Haggis controlled him to attempt to destroy Hairy Worm.