Not much is known about Marnier's personality, however, since she took part in Big Mom's plan to assassinate the Vinsmoke Family, she can be consider very loyal to her family and crew. She was frightened by the thought of Big Mom reaction to the destruction of Mother Carmel's photo.
As the Minister of Yeast, Marnier has authority over Kibo Island, and as an officer of the Big Mom Pirates, she has authority over lower-ranking members of her crew. Her combat powers and abilities are unknown, but she is presumably strong as she is an officer of an Emperor-led crew.
After Brook broke Mother Carmel's portrait, Marnier worried about Big Mom's reaction. She later joined her crew as they closed in on the alliance. When the Vinsmokes tried to provide cover for the alliance, she fired at them.