Polly (aka. Polly Wright) was played by Anneke Wills from 1966 to 1967.
Secretary to some fourth wall-breaking super computer, Polly stumbled onboard the TARDIS along with Ben as an unwilling traveler. Curiously, the two of them were rather blasé about possibly never getting back to their own time again, probably because the BBC execs told the production team "look, we've just come off of two old farts constantly whinging about wanting to go home, we don't need another pair of assistants to continue that trend alright?". And so, they only really mentioned the prospect of going home like once or twice.
Like Ben, Polly appeared in 9 stories (36 episodes) but the only story featuring her which is currently complete in the BBC archive is her first, The War Machines. She fucked off together with Ben in The Faceless Ones.
After leaving the TARDIS, she and Ben ended up getting married and running an orphanage in India (or not, depending on whichever form of non-canon spin off media you get a boner from).