Doctor Who: What Happened To Every Companion?
7. Colin Baker Era
Colin Baker's era has one of the smallest number of companions in the entire show, and only one who actually left during his tenure. So with that, let's talk about Peri Brown.
Peri was a college student who originally joined up with 5 after Turlough left (he's even the one who introduced them). They got along decently enough, until he regenerated into 6, who immediately showed himself to be an arrogant braggart. The two butted heads constantly, though there was definitely a slight respect between them under the surface of it all.
Her ultimate fate is...tricky. Long story short, The Time Lords tried to get rid of her in order to cover up a conspiracy from The Doctor that she found out about. Because so many warring factions of Time Lords were fighting over what to do with her, this caused Peri's fate to splinter off into hundreds of alternate timelines. Different expanded universe material has given different accounts of what ultimately became of her, but due to her circumstances, they're all technically true.