Bat Within activates when Bayonetta is hit by an attack early during rolls, sidesteps, or air dodges, or late during Witch Time. Nimble/Dodgy Dodger Equipment does not affect this window. When this happens, Bayonetta transforms into a flock of bats, halves the amount of damage inflicted, and continues moving in the direction she was traveling; for a roll, she will advance in the same direction, and for a sidestep or after Witch Time, she will stay in place. She can travel in any direction if done during her air dodge, but needs to have jumped prior if she wants to rise with it.
When Bat Within is activated, she endures some ending lag, meaning that it is possible to extend a combo on her if it was activated by a faster attack or a long-lasting, multiple hitting attack. While Bat Within makes Bayonetta's rolls, sidestep and air dodge appear to be the best in the game, each of them possess noticeable ending lag. To compound this, several characters, such as Duck Hunt and Mewtwo, have rolls, sidesteps, or air dodges that start up just 1 frame later without being nearly as vulnerable at the end.
Bat Within also has the distinction of being treated as a counterattack that does not strike back; therefore, it does not trigger against grabs, Final Smashes, or unblockable attacks. Although difficult, it is possible to simultaneously activate both Witch Time and Bat Within if Bayonetta is struck precisely when their activation windows overlap. However, doing this is actually a hindrance, as she will still receive halved damage, while Witch Time's slowdown effect on the opponent is reduced.
Bat Within is a technique purchasable from the Gates of Hell. By dodging at the last second, Bayonetta instead activates Bat Within, transforming her into a swarm of bats to escape, while also negating the damage taken from the attack and slowing down time for an extended duration, unlike in Smash.
The way Bat Within works in SSB4 during Witch Time is the opposite to how it works in Bayonetta; it instead activates as Witch Time ends, and reduces slowdown time if both are activated instead of increasing it. Interestingly, Bat Within triggers in the first four frames of her air dodge and sidestep, and from the 3rd to 5th frame of her roll, which is more in line with the technique's original intended use, though in every case, it only halves the damage she would have taken, rather than completely negating it.