Miya had accompanied Nerva and Tanya to look for Sasha's parents, she was there to protect the two young girls as both didn't have any combat ability.
Nerva was still too young that her body strength and coordination hadn't matured yet, even though her ability could make her unbeatable in a fight with those on a similar level of power, currently, almost everyone has more power than her.
The heir of Minerva's power was overpowering in its own way, Nerva could predict anything she observes as long as she had enough information she would never get caught by surprise, being able to move a view second in advance was very advantageous in a fight.
Tanya's ability was also great, she could compromise her opponent's judgment or motivate her ally with her word but she was not trained for combat, she would need someone to fight for her instead.