One of the rules for companions is that they shouldn't be as intelligent as the Doctor. The companion is the audience's proxy for most of the show, so when the Doctor explains something to his companion, he's explaining it to the audience at home, too. But that doesn't mean that they should be stupid. Introduced near the end of the fifth Doctors tenure, even the production team freely admit that Peri was created as eye candy in order to draw in a larger number of teenage and adult male viewers. The above image says it all, really. Peri spent her time with the Doctor screaming and getting kidnapped while dressed in an array of revealing costumes, thus leading the Doctor into a series of dangerous environments to rescue her. Possibly worst of all, the character was American, even though the actress playing her wasn't. So not only was she extremely annoying, but she was extremely annoying with a bad American accent. Far from acting as the surrogate for the audience, Peri did little more than piss everyone off by stumbling around asking a series of inane questions. Disappointingly, Peri did not depart the show in a bodybag. In one of the most unfair turn of events ever, she lived happily ever after as the bride of a warrior king on an alien planet.