Nihu

Nihu is a former Toa Mangai of Water. She is thought to be dead by most of the universe.

Matoran
As a Matoran, Nihu lived on the Tren Krom Peninsula. Eventually she became a Toa.

Toa Mangai
Many years later, Nihu was called to Metru Nui to fight against the Kanohi Dragon the Dark Hunters had unleashed. After a month-long battle, she decided to stay and guard Metru Nui, despite being given the option to return to the Tren Krom Peninsula.

Teridax's Plan
while Teridax posed as Turaga Dume, he sent Nihu, along with another Toa mangai, Motara, on a mission to Xia. There, several Vortixx and Brotherhood Agents tested an Energy Siphon Blade on them, which was later named after her and Motara.

Due to an accident with the prototypes of the weapon, the two Toa lost their elemental powers, which were replaced with energy-draining abilities and the ability to temporarily steal other being's powers. Fearing Teridax, the Xians used Brotherhood technology Teridax lent them to hide the Toa and the prototype in an alternate universe, telling him they were dead.

What happened to Nihu since then is unknown.

Abilities and Tools
As a Toa of Water, Nihu could create, control, and absorb water. Since the accident on Xia, she no longer has these powers.

The energies used in the prototype have integrated themselves into Nihu, giving her the ability to drain energy, and steal other being's powers. The stolen powers wear of after a short while however. These powers do not work on Makuta or Motara.

Nihu's original weapons were a mace and sheild, although these were confiscated on Xia. She now uses on of the prototype Nihu-Motara Energy Siphon Blades that were disposed of along with her and Motara. Her mask is unknown.

Trivia

 * Nihu was inspired by the Nihu-Motara Energy Siphon Blade, which can be seen on BS01.
 * Storywise, the Nihu-Motara Energy Siphon Blade was named after her and Motara.
 * Nihu's power-stealing abilities were inspired by Kirby, althugh she does not have to eat anyone to use them.