Celestian
Celestians are the elite warriors of the Sisters of Battle, veterans with years of experience who have proven themselves to be the best that the order can provide. They are sisters who have looked evil in the eye and laughed straight at it and schooled it seriously.
Outside of battle they assist with the day to day running of the order with the Canonness and their senior members are called senior celestians. They act as liaisons between the sister superiors and the canoness.
While they are cool and the kind of women that should inspire ideas of strong female representation within Warhammer 40,000, unfortunately all too often they are either thought of in depraved ways or beaten up by other armies to prove the superiority of something lame in their own codex.
For example when an Inquisitor found the Sons of Malice to be engaging in barbaric rites she mobilised an entire company of Celestians to assault the company. The marines didn't take too kindly to this and after killing all the Celestians despite being surprised and ambushed, they ritually ate the Inquisitor too.
Lots of strange Fruedian undertones going on here then.
Come on GW for once just let the Celestians and the rest of the Sisters trounce someone please.