She is a former colleague of Randy Orlando. On the surface level, she addresses him as a good for nothing and happy-go-lucky guy. However, she deeply cherishes the brooch he gave her as a congratulatory gift for her promotion.
During the Cult Incident, Mireille and her team are manipulated into attacking the IBC building. After the incident gets resolved and Commander Douglas was demoted, Mireille was promoted to second lieutenant under Sonya Baelz as the new commander. She supervised a rehabilitation training in the Knox Woodlands over the summer that followed.
When the Crossbell Guardian Force changed into the Crossbell Defense Force after the declaration of independence, Mireille acted as the leader of a resistance that questioned the new military structure. As Crossbell's independence and its Defense Force were declared illegitimate, Mireille's rebellious attitude was overlooked and reinstated by Sonya.
She assisted Sonya in maintaining order among the confused citizens as the Azure Tree appeared and disappeared.