None of these defibrillation robots (except for the reject one) are seen, but their purpose is to revive passengers that have suffered heart attacks. The reject D-FIB appears to have too much charge in her paddles, shown when she shocks a test dummy and sets it on fire. This comes in handy when battling with the SECUR-T bots. She is freed from the Repair Ward by WALL•E and helps fight the SECUR-T bots by frying them with her overcharged paddles.
D-FIB also appears in the WALL-E: The Video Game where she unlocks security doors (with an image of her) for WALL•E.
The reject D-FIB has a broken arm (due to the way it hangs down).
According to the film and the video game, the only word she says (and can say) is "clear".