|“||I don't feel properly dressed without a couple of cans of nitro.||”|
She's a 20th-century Earth teenager from Perivale, who got warped to an ice planet where she boned Sabalom Glitz.
The most based companion. No flirting, no whining, no screaming, no fucking around, she just pops in, kicks some motherfucking ass and leaves. Also, she's hot as fuck and would probably have a threesome with you and another girl.
She was known to have many personal flaws, which the Doctor made her face by often taking her to places where she had been traumatized or to meet people who she hated, as a means to get her to face her insecurities and be more mature (it's a shame he never did this to a certain other chav companion). Apparently this was so he could make her good enough to become a Time Lord or something.
Bill murdered her and stole her outfit.
Ace met the Doctor on Ice World in 1987's Dragonfire, where she ended up replacing Mel as his companion (thank fuck). She continued to travel with him for the remainder of the Classic Series, still being onboard in 1989's Survival.
Since she was never actually shown to leave onscreen, her departure from the Doctor is a subject of heavy debate among hardcore fans who have nothing better to do with their lives than theorize the exit of a fictional character from a TV show. The expanded universe has come up with a seemingly infinite number of circumstances where she ended up leaving him, either actually becoming a Time Lord as intended, being returned to Earth to run a charity, abandoning the Doctor because he was responsible for the death of someone she was in love with, or being killed in an explosion.
Who's to say she ever left though? My headcanon is that she's still onboard to this day, wandering the TARDIS corridors and having offscreen
sex banter with the Doctor and whoever the current companion is.