Walter Plinge
Knock-kneed buffoon and general dogsbody at the Ankh-Morpork Opera House, Walter wears a beret and has absorbed opera until it subsumes his soul. For he harbours a secret, does Walter, underneath the beret that was made for him by his mum, Mrs. Plinge. All becomes clear in Maskerade, but if you don't mind the plot being spoilt then read on...
Walter is only composed of elbows and knees when he's being himself. When he slips on a faceless mask he becomes the second Opera Ghost. Not the evil one who kills for money, but the one who sings like an angel and composes a whole series of a new style of musical entertainment, based more-than-loosely on the hit musicals of Roundworld's Andrew Lloyd Webber.
He is one of the few characters in the Discworld novels to come away from an encounter with Granny Weatherwax without being permanently damaged - indeed, he comes away headologised with a far brighter future: no longer the errand-running figure of fun whose mother worries about him but the new Director of Music at the Opera due to the death of Mr. Salzella.
Like the other people in the Opera house Walter has a certain degree of Opera-connected madness, but unlike everyone else Walter is actually connected to Opera (the house and music). The connection is so strong and deep that Granny described the connection has two entities being fused together when she reached out in to the Opera with her mind and brushed Walter's mind. Because of this both of Walter's personalities have a deep love of the Opera house and everything related to opera. So much so that the Ghost's lair is filled with old and discarded props and costumes that he spends time repairing. The lair is also beneath the stage which allows the music to filter down and "fill the soul". Walter's connection with Opera can also cause him pain as when the Opera was stopped before it finished Walter was physically and mentally hurt by it knowing that the show had to be finished.
Annotation
It may be superfluous to mention this, but the character and appearance of Walter Plinge owes a lot to the character of Frank Spencer, in long-running BBC sitcom Some Mothers do Have 'Em. (The mother in question might have been a despairing Mrs Plinge.) Spencer is a gormless bungling child-man, who gets himself into strange and inventive situations requiring the full armoury of pratfalls and physical humour to get himself out of them, usually with much destruction happening all around him, from which he escapes unscathed. He generally affected a shapeless trenchcoat and beret, and was played by one Michael Crawford, who incidentally went on to international stardom in a musical called The Phantom of the Opera. Crawford went from shambling cloddishness to physical grace and presence in one very smooth transition, almost as if donning a mask effected the transformation...
It is also said by his mother that the young stagehands at the Opera will play cruel tricks on Walter, like hiding his broom. This evokes the film The Last Picture Show, which follows a group of young American school friends through graduation from high school into adulthood in a tiny Mid-Western town where tumbleweed really does blow down the main street. One of the group, who will never see adulthood and is fated to be an eternal child, is what used to be known as a "retard", a simple boy who will actually go out with a broom and seek to sweep the street. Generally speaking, his peer group are protective of him, but he is the butt of unthinking cruelty on occasion...
Walter's desire to keep on cleaning the toilets even after his promotion to music director is reminiscent of Stanley Spadowski, another ungainly man-child janitor whose unlikely talent is discovered under silly circumstances in the 1989 film UHF.