His name means one thousand in French.
The youngest of the twins born to Zechs Merquise and Lucrezia Noin, Mille has his father's blue eyes and his mother's raven hair. He is left-handed and is described to be taciturn. On occassions he does choose to speak, Naina always seems to beat him to it. However, he has been shown to have a sense of humor despite his reserved nature. Mille is musically-talented and is an excellent flautist and pianist.
In his POV chapter, Mille admits to constantly feeling restless and that he finds it difficult to express his feelings and thoughts through words. He also dislikes conflict and has actively avoided them growing up, believing that as long as he is safe he is fine. Mille proves his strength when he pilots his father's suit, Tallgeese Heaven. He is quick to grasp the controls of the suit and step up to its exacting demands.
Naina and Mille are very close and work well together despite their seemingly opposite personalities. One is rarely seen without the other, although they did spend some time apart when they were little. For two years, Naina stayed at the Schbeiker Orphanage for her safety. In that time, it was assumed that Mille remained with their mother as she acted as guard for The Little Prince.
Mille and Quaterine are attracted to one another and as two sensitive types who are also musically-inclined, they seem to understand each other well.
The two first meet at a dinner that the newly awakened Relena Peacecraft hosts. After some tense exchanges with Naina, Mille goes over to her and asks her to play the violin along with him.
Before defecting to the Mars Federation, Quaterine hints to Trowa Phobos that she may have fallen in love. It is unclear to whom, as while she seems to be fascinated with Phobos and even kisses him in order to distract him so she can make her getaway, she quickly apologizes to Mille when the latter and Naina picks her up. Mille does not know the context of the apology as Quaterine never says it out loud, but it is for him (Mille) not being her first kiss.
Approximately four Mars years prior to the events of Frozen Teardrop, Master Chang presented himself as a teacher to young Mille and gave him martial arts and MS training.
Aware of Master Chang's history with his mother, Mille does not quite understand why the man would offer to take him under his wing and wonders if it was only a ruse to be able to investigate the Mars Federation. Regardless, Mille regards Master Chang with utmost respect and is grateful for his strict guidance although he does worry if the latter still see him as weak.
For his part, Master Chang displays his care for the boy by saving him when his suit is shot down by Black Wing.