Block the effects of other mental abilities
- "Her mind is impenetrable; no one can read her thoughts unless she allows it. She can shield herself from all types of psychic attacks and learns to shield those around her."
- -Entry on Bella's gift
A Mental shield is Bella Swan's raw supernatural force-field that blocks any and all kinds of powers acting on the mind, not necessarily malevolent ones. Aside from Bella, her father Charlie shows a very similar power, though very rudimentary and undeveloped compared to Bella's.
As a human, Bella considered this power a 'glitch' in her head; Edward had teased her that he could only read FM minds, and that she was on AM. Her shield seems to be based on her "private mind", a place where no one can harm her. It also made her very desirable to Aro, who likes to collect vampires with special abilities for the Volturi guard. It is known to be unbreakable to psychic powers.
Three months after her becoming a vampire, Eleazar correctly describes it as a shield. With the help of Eleazar, Kate and Zafrina, Bella learns to gain some control over it, eventually coming to include a number of other individuals in her shield at the same time and protect them from psychic powers.
This proves useful in Breaking Dawn when she includes her family and friends under it, protecting them from the powers of Jane, Alec, or any of the Volturi guard. She also learns to remove it to allow mental powers to work on her, and to allow Edward to read her mind. While her main shield is pushed out to protect others, a secondary shield also surrounds her personally. She can manipulate it in any way she pleases, either by wrapping it around her targets like plastic or turning it into a giant dome. It also works as one-way glass, allowing the powers of the shielded vampires to work undisturbed. Bella mentions that she can sense those she is shielding, since they somehow become part of her. In the case of shape-shifters, a cascading effect has been observed: shielding an Alpha ensures the protection of the entire pack. In battle, Bella needs to be protected from physical attacks in order to keep her shield around her allies.
Bella initially had a very hard time pushing it out, describing it as an elastic recoil. This obstacle mainly comes from her subconscious desire to protect her mind above all else. Once she learns to let go of that desire, she gains complete control over it, able to shape it in any way she pleases (even make it wrap around her targets like living plastic), though it takes concentration to make it work properly, even make it follow the movements of those she protects. This comes in handy when she removes Kate from her shield while still protecting Garrett.
At the end of Breaking Dawn, Bella removes her shield to allow Edward to read her mind. She describes this as much more difficult than pushing it out, as it struggles to protect her and it takes all of her concentration to make it disappear altogether.
Though this shield is powerful, it has many limitations. For instance, it cannot work against a physical attack, allowing for an intruder to attack directly. Also, if the intruder has a power, then once he is under the shield, his power will work effectively against anyone behind it, except for Bella, since she has a second shield specifically designed to protect herself.
Renesmee Cullen is the only individual capable to penetrate it, as she can place thoughts and images into her mother's mind.
Her shield is impotent against a power that is not invasive to the mind. Alice Cullen manages to get around it, because her ability allows her to sense the outcomes of decisions already made, which would probably relate more to the flow of time than to the mind of individuals. Bella also can't block Jasper Hale's ability to control emotions; Marcus's ability to read relationships as seen when he reads the love between her and Edward; Siobhan's ability to affect reality; Benjamin's control of natural elements because it's a purely physical power; James's tracking sense; and Fred's ability to physically repulse others.
- Renata, Aro's personal bodyguard in the Volturi, has the power to repel physical attacks, much like how Bella's shield repels mental powers. However, it is limited by making the pursuer want to go in another direction.
- Afton has the power to shield himself from attackers by making them think he is invisible. His power is more limited in that he can only shield himself and can be seen through by sharp focusing. Since his power only makes others think he is invisible, this power most likely works on a mental basis.
- Fred has the power to induce those around him with a sense of revulsion to drive them away. Those power serves a great purpose in protecting him from pursuers.
- Renesmee Cullen shows an opposite version of this ability, which allows her to penetrate Bella's shield, or go unaffected and use her power of tactile thought projection on her.
- Charlie Swan is also capable to shield himself against mental powers, along the same lines than Bella's, but a lot less completely.
- Danutza Roman has a similar-different ability and it is called Physical Shield. It protects from physical weapons/attacks.