Saint Celestine

"'That you cannot be condemned for. Without you, Fyodor Karamazov would not have shaped up to be the bitter, dogmatic and furiously self-assured vessel that he is today.'"Saint Celestine is the most well-known Living Saint in the Imperium of Man, playing an active role in the turbulent 41st millennium.

Official Canon
Celestine was a Repentia of the Order of Our Martyred Lady who fought in the Palatine crusade. During the fighting, it soon became apparent that she had been blessed by the Emperor, though it wasn't until she recovered the amour of Saint Katherine that she was officially declared a Living Saint.

Even after her seemingly apparent death, she'd appear from the Warp to aid Imperial soldiers in their hour of greatest need, surrounded by heavenly light and descending upon a giant pair of wings.

Celestine was actively involved in the defense of Cadia during the 13th Black Crusade, helping the surviving Imperials to escape into the Webway, and was present for the revival Roboute Guilliman at Macragge.

TTS Canon
Celestine appears alongside Dominique's soul and the Star Child as they find Fyodor Karamazov trying to get past the Gate of Khaine. Celestine and the Starchild had plans for Fyodor and made sure that he would escape from the Warp.

She appears again in the video Salvation Through Annihilation, by Eliphas, where she saves a Shrine World from Ku'Gath the Plague Father.

Celestine revealed to the Blood Angels after decapitating Be'lakor that she and The Sanguinor like to switch up roles periodically to keep things fresh. She also accidentally called the Sanguinor her "Boy Toy," effectively outing their relationship.