Mio Nagumo is a main character in the 7th Dragon III Code:VFD.
Mio travels to Tokyo in 2100AD; UE77 to find her supposedly dead father and encounters the protagonist who was attempting to enter Nodens 7th Encount virtual reality game but was stopped by Nagamimi for not having a ticket. Mio notices this and offers to go inside together with the protagonist as she is too nervous to enter alone; she presents her special S-Rank ticket(s) to Nagamimi who immediately allows them to enter and together with the protagonist, they enter the game's lobby.
Mio acts as the protagonist's Navigator while playing 7th Encount, but is unseen on the screen. As the protagonist progresses through 7th Encount, they come across a Dragon coded within, and Mio states she can tell it is a powerful monster just by its features. This comes as a surprise to her; she doesn't know how or why she has this knowledge.
After playing 7th Encount with the protagonist, Nagamimi tells the protagonist and Mio to follow them to Nodens Headquarters. After some prodding, Nagamimi explains to Mio and the protagonist that they both hold the ability to take down Dragons - the protagonist as a Dragon Hunter, and Mio as a Navigator. This frightens Mio, and she states that she is not fit for saving the world, and leaves Nodens.
Once outside, Dragons descend upon Tokyo, and Mio cowers in fear at them. The protagonist rushes out to save her, and keep the Dragons at bay until the ISDF arrives. After all of this, she still leaves Nodens, but returns later on to take over for Nagamimi as the game's main Navigator.
Later, Unit 13 finds out that Mio is actively dying of Dragon Sickness, but still holds the determination to help the Unit in their travels across time.
Mio Nagumo on the Heroes wiki.