As a partner character
For much of Resident Evil 4, Ashley will be by Leon's side - his job is to protect her, after all. However, being a university student, Ashley is not trained in combat and is useless in battle. Therefore, she must be called to "wait here" while Leon advances, or "hide" in nearby crates until Leon whistles which signalling all-clear. If Ashley is grabbed by a Ganado, he will take her to the nearest door. If she is taken out of the room, you will lose the game. You must, therefore, take priority in killing the Ganados near Ashley. Should she be captured, shoot the Ganados in the legs - you cannot risk hitting Ashley, as any hits from gunfire or knife-slashes will lead to an instant death and game over.
While wearing her second alternate costume, unlocked after beating Seperate Ways, Ashley is unkillable due to the armor she wears, not a single attack can hurt her (including explosions). Also, when Ganados attempt to kidnap her, they will fall over due to the weight of the armor. Instead of ducking when Leon aims in her direction, Ashley simply lowers her visor. Unfortunately, this can be a hinderance as Ashley can often block Leon's shots.
Playing Ashley
In Chapter 3-4, Ashley will be controlled by the player as she navigates the castle in search of Leon. There are no firearms or knives to be equipped and used, however, so Ashley must largely run away from the robed cultists. Oddly enough, these enemies (and the armaduras encountered later) won't try to capture Ashley. Instead, they'll either strangle her to death or kill her with their axes.
In the first two rooms, however, she can throw explosive lanterns at nearby cultists -or even both if together-to kill them. If she misses a cultist at any point, you must leave the area and return in order to trigger a respawn.
Interestingly, Ashley can suplex Zealots if she hits them when opening a door. This only happens in the GameCube version.