A new permanent character added to a TV show as an attempted improvement, which ends up backfiring by changing the chemistry of the show altogether and destroying whatever momentum the show already had.
Polar opposite of an Urkel (where a terminally ill show was revived by adding the new character).
Giggle and Hoot (Australian Childrens TV show on ABC) were a perfectly functional TV team presenting a "wrapper show" (i.e. wrapped around short kids shows). The chemistry between Jimmy Giggle (cool guy) and Hoot (owl puppet) was great until ABC felt they had to add a female counterpart, "Hootabelle". Chemistry gone. Dynamic duo no longer dynamic.
The Big Bang Theory started with outstanding chemistry by juxtaposing four awkward nerds with one hot female neighbour. Two key elements were Howard's sleaziness (thinking of himself as successful with the ladies but ultimately failing repeatedly) and Sheldon's uniqueness (no further description required). This show was running hot - and yet the writers added not one but TWO Poochies - Bernadette (now Howard's fiancee) totally neutralises the failed Casanova element that made Howard funny, and then Amy Farrah Fowler comes along as a "female version of Sheldon", removing all that was unique and hilarious about the show's strongest character. Series 4 and 5 TBBT are materially different from the earlier episodes that made this show a world-beater.