SPOILER WARNING: This article contains major spoilers for "Star City 2040," the latest episode of Arrow.
Oliver may have been absent from Mia's childhood, but Felicity didn't raise their daughter alone. As revealed in "Star City 2040," the latest episode of Arrow, Felicity enlisted a little help from Nyssa al Ghul.
The episode handily answered the question of Mia's training. As it turns out, Nyssa was a constant in Mia's life as she was growing up. Through a series of flashbacks, Mia went toe-to-toe with Nyssa through various stages of her life. Finally, when the character aged into becoming Katherine McNamara, Nyssa gave her a seal of approval and concluded the girl's training.
The daughter of Ra's al Ghul, Nyssa debuted on the series in Season 2 and remained a consistent presence. However, she began appearing less frequently after she disbanded the League of Assassins. Indeed, she appeared a handful of times in Season 5 and only once in Season 6, though she has appeared in 19 episodes overall.
Nyssa's inclusion in "Star City 2040" was a brief surprise. She appeared in the first few minutes of the episode and left once Mia's training was finished. At the time of writing, it is unclear if Nyssa was a constant presence throughout Mia's life or if she simply reappeared from time to time. Regardless, her involvement in Mia's life certainly paid off, as Mia is an accomplished fighter by the time of her introduction.
Airing Mondays at 8 pm ET/PT on The CW, Arrow stars Stephen Amell as Oliver Queen, David Ramsey as John Diggle, Emily Bett Rickards as Felicity Smoak, Rick Gonzalez as Wild Dog, Juliana Harkavy as Black Canary, Colton Haynes as Arsenal, Kirk Acevedo as Diaz and Katie Cassidy as Black Siren.