Sofia Fox (Season 1), Madeline Rose Yen (Season 2-present)
Milli is the only female, leader of Team Umizoomi and older sister to Geo. She is the primary protagonist of the series. Also she is the only known relative of Geo.
Milli has red hair held in ponytails(or pigtails) and rosy cheeks. Without her helmet, her ponytails are bigger and fuller. With her helmet, her ponytails are flat and ribbon-like. She mostly wears a dress and a helmet, tight stockings and Mary-Jane shoes. The outfit's default colour is pink and the dress and helmet have a pattern of pink flowers. The outfit changes colour and pattern (See Milli's Many Patterns). Sometimes when someone or Milli mentions her dress, she curtsies for fun. She has definite age to be 6 years old, according to the website.
Milli-Measure: Milli's pigtails, but she prefers "ponytails", become a ruler, scale, Etc. She can also use it to add measurements to specific things, which was revealed in one of their promos.
Pattern Power: In episodes up to "The Big Boat Race", Pattern Power is, basically, applying a pattern onto an item. In episodes starting with "The Dinosaur Museum Mishap", Milli changes her dress into a pattern mostly to add somthing missing to the pattern by peeling the picture of the item off with that picture fading then more colour added, to fix a pattern or to follow a pattern to get through obstacles. Examples of her dress patterns are on Milli's Many Patterns.
Ponytails: Milli's hair extends to her liking. See more on Milli-Measure.
Karate: Milli knows how to do karate. She has powerful chop, kick, even her ponytails. She mostly uses her skills to get past pattern and remove measurements or to divide something into fractions.
Milli is really sweet, cute, and generous, but don't let her cute looks and sweetness fool you. She is also fearless, strong, and would do anything to help and save her friends. That's why she's the leader of Team Umizoomi. She's also the strongest member of the team.
Even though Milli really loves and cares for her brother Geo, she sometimes doesn't have faith in him with his ideas at first.
Revealed in The Kitty Rescue, Milli has a pet bunny named Whispers. In Umi Toy Store, Milli revealed to have some experience in playing the toy piano. Her favourite food is her Pattern Sandwiches. Also her favourite animals are bunnies.
Milli is known as the animal-lover on the team. She just can't stand the sight of baby animals so much, that she can jump for joy. Her love for animals make her a great rider of animals. From horses, to dinosaur, to even dragons, Milli is not afraid to ride any animals, and she always asked before getting on animals.
It is known is that Milli was born before Geo. It is not known how her ponytails can grow and shrink and how her dress has the ability to change patterns. It is not revealed to how she and Geo met Bot. Her parents and other relatives are not revealed as well. However, only reletive that was revealed was her brother, Geo.