Berthier

ベルチェ

Berthier (known as Bertie in English versions, though sometimes implied to be Birdy) is the second-youngest sister (the youngest in the dub). Her name is French, derived from the mineral berthierite, and is pronounced roughly as "BEAR-tyay." Her magical attack is called "Dark water," and she serves as the counterpart to Ami Mizuno. She fears Rubeus but, unlike her other sisters, has no romantic interest in him.
In the manga, Berthier is assigned the mission "Code002, Operation Re-move" to kill the Sailor Senshi and capture Chibiusa. She disguises herself as an ordinary human and impresses people with her dowsing abilities using a pendulum. When she observes Ami Mizuno locating a leak in a pipe without using a pendulum, she deduces Ami's identity as a Sailor Senshi. Skilled at chess, Berthier appears on television to challenge Ami to a match. During the game, she addresses her as "Sailor Mercury" and attempts to use the Black Crystal to manipulate and capture her. A battle ensues between Berthier, Mercury, Jupiter, and Moon, during which Berthier proves powerful enough to deflect Mercury's Shine Snow Illusion attack. Finally, Berthier uses water from a sewer to surround and choke Mercury, enabling Rubeus to abduct her. Berthier is subsequently killed by Sailor Moon.
In the anime, Berthier displays a very darkly playful personality but shows great respect for all her siblings. She is the only one depicted as upset when Cooan is purified by the Silver Crystal. Much like her counterpart Ami, she is also a strategist and incredibly talented at chess, though she apparently cheats. On her final mission, Berthier plays a deadly game of chess with Sailor Mercury, but her plan to freeze them all fails when Petz and Calaveras arrive and inform her they will claim credit for her success regardless of the consequences for her. Momentarily distracted, she loses her chance at the killing blow against Sailor Mercury when Tuxedo Mask arrives and rescues her and the other Sailor Senshi. Her two older sisters then abandon her as a failure. Distraught, Berthier decides to sacrifice herself to kill the Sailor Senshi and uses all her power ("Dark Water! Full Power!") in one final blizzard. However, a newly healed Cooan convinces her that she is still loved and has a second chance. Berthier abandons her suicidal gambit and allows Sailor Moon to heal her. Later, she and Cooan, along with the Sailor Senshi, manage to convince their other two sisters to join them and become normal women.