D'artagnan

Professor D'artagnan, also spelled as Dartagnan, is a professor at a university in England with Seitaro Takamine. Despite appearing as a serious-looking man (and he is a serious guy), D'artagnan has a habit of disguising or cosplaying in odd costumes.

He was first introduced wearing a sea bass costume while smoking a cigar when Kiyomaro bursts in to his office. He tells Kiyomaro to forget what he saw. He is later seen in a fairy costume saving Gash from snakes saying he is the spirit of the forest and investigating the explosion that happened in the forest six months ago. He is later seen in a helping Seitaro with the stone tablets.

He appears in later episodes of the anime, wearing outfits based on other people's clothing regardless of their age or gender and acts into their persona.

(Source: Gash Bell Wiki)