Humphrey is a story with two main purposes. First it was intended to begin the transition of those relationships, Bo slowly becoming more attached to Sammie than Chrístõ. The man to man talk Chrístõ has with Sammie sets out the terms clearly. Chrístõ is NOT handing her over to him, but he is not going to stand in his way.

Meanwhile, to the caves of Betalon 8, with Chrístõ and Sammie both searching for Bo. Sammie finds her first, and for a while there is a connection between them, but reunited with Chrístõ she is in no doubt where her feeling lie – for now. All well and good, and having seen enough of Betalon 8, they take off again, only to find that they have a stowaway.

Humphrey Boggart. An easy joke, of course. The fact that Sammie comes from Wigan actually came in useful at this point. It wasn’t planned, but it happens that the idea of “boggarts” are associated with the East Lancashire moors around that part of the country. They are NOT shape shifters as imagined by JK Rowling in Harry Potter, but they are malevolent and secretive spirits. For anyone of a certain age, Humphrey is also a name associated with invisible spirits. For several years in the early 1970s milk was advertised with a slogan ‘watch out, there’s a Humphrey about’ and scenes of straws being carried by unseen creatures. The connection with a certain Holywood actor is mere coincidence.

What Humphrey ACTUALLY looks like was inspired by one of the monsters in an old DOS-based computer came called Moraff’s Dungeon, which had strange puff-ball creatures that were either good or bad depending on their colour. Humphrey was based on one of those, the good ones in his case.

Chrístõ’s injunction on him not to go into the bathroom or bedrooms goes without saying, but the TARDIS had acquired a new and strange crew member.

http://en.wikipedia.org/wiki/Boggart