The campaign has, for some time now, had only two player characters: the Doctor and a single female companion. Which isn't really enough for an RPG. So, when, part way through the eighteenth season, the annoying kid brother of one of the other players demands to be allowed to join in... well, what are you going to do?
The new player, of course, creates alien boy genius Adric.
Now, we just have to face reality here. You're never going to get a group as large and diverse as Doctor Who fandom to agree on anything as controversial as the identity of the "worst companion ever"... but, the fact remains, if you look at just about any list ranking companions by popularity, Adric is going to be somewhere in the bottom three. He might not always come last, but he is not, by any stretch of the imagination, a popular character.
But let's take a step back from that for a minute, and just consider his character sheet. What, exactly, is the point of Adric, and how would he fit in an RPG campaign? On paper, at least, the answer is surprisingly well.