Acid

Acid (アシッド) (real name unknown) is a member of Blue Blood and Counselor's twin sister.

Appearance
Acid looks identical to her twin Counselor; a short girl with silver hair tied in a ponytail, wearing a baggy dress shirt and leather boots, with her bare legs shown.

Toaru Kagaku no Railgun SS3
Acid and Counselor accompanied Sakibasu Ryouu to the building holding the corporate server of Zero-Day Link, and were tasked with preventing anyone from passing while she infected the server with Blue Blood's virus. They fought against Misaka Mikoto, Shokuhou Misaki, Shirai Kuroko and Hokaze Junko, and caused the group some trouble together, but were eventually defeated. After the fight, the two were bound and gagged before being put into random lockers.

Abilities
Acid is a Level 4 esper whose ability allows her to control oxygen. Her initial control is limited to pure oxygen and rather than gathering it directly from her surroundings with her ability, she acquires it through inhalation, producing it from the air in her lungs. After exhaling the pure oxygen under her control, she can manipulate it in a variety of ways.

Aside from shooting concentrated oxygen and producing gusts of wind (which is noted to be a somewhat inefficient use of her power), she can use it to influence the air to bend light in a manner similar to a mirage and throw off an enemy's aim. She can combine the oxygen with hydrogen ions and then airborne moisture to form a strong, as well as neutralizing the harm from it around her. She can also combine the oxygen with other substances in the air, such as carbon and nitrogen, to form blocks of oxides.

As well as exhaling the oxygen under her control to manipulate outside of her body, Acid can take the oxygen into her body to manipulate it, controlling the flow of oxygen in her body down to the cellular level to place her body under her full control. During her clash with Hokaze Junko, she took in enough oxygen to cause hyperventilation under normal circumstances and displayed a boost in strength in a similar way to her enemy's Rampage Dress.