He is a (living) statue of an anthropomorphic cat who is fully dressed (including a top hat and shoes). At night, he comes to life. He belongs to the owner of an antique shop named Shirō Nishi.
Due to his popularity, Baron was given a bigger role in The Cat Returns, where he, along with Muta, aids Haru Yoshioka as she is unwittingly brought into the Cat Kingdom. The Baron works in the Cat Bureau.
In the original Japanese versions of Whisper of the Heart and The Cat Returns, he is voiced by Yoshihiko Hakamada and Shigeru Tsuyuguchi, respectively. Cary Elwes provided the Baron's voice in the English release of The Cat Returns, and reprised his role when Whisper of the Heart was dubbed two years later. Elwes interpreted The Cat Returns as a prequel to Whisper of the Heart, and chose to play the Baron as much older and more refined.
The baron has an Old German name. His first name Humbert came from hun "warrior" and beraht "bright". It also came into use as a surname and as noble rank.
His surname is von Gikkingen. von also implies his noble origin and Gikkingen could be a fictitious location.
Baron: I'm Baron Humbert von Gikkingen. The Person who created me named me so
- Whisper of the Heart - Behind the Microphone. DVD Extra