Looks like my previous guess about Katana was on the money.
Deadline announced late yesterday that actress Devon Aoki has been cast as Tatsu Yamashiro, better known to DC Comics fans as Katana, for the upcoming third season of the CW series Arrow. According to the article, Tatsu will be "one of Oliver’s mentors in the (Hong Kong) flashbacks and a critical influence on his journey to eventually become The Arrow of present day."
Aoki is best known as the mute Japanese assassin Miho in the 2005 film adaptation of Frank Miller's Sin City. Her half-brother, Steve Aoki, previously appeared in the Season One episode "The Huntress Returns," playing himself at the opening of Oliver Queen's nightclub Verdant.
Created by Mike W. Barr and Jim Aparo, Katana first appeared in The Brave and the Bold (vol.1) #200 in 1983. Skilled in martial arts, Katana was married to Maseo Yamashiro until Maseo was killed by his brother Takeo with a sword, a katana called Soultaker, as their children are killed. Tatsu took possession of Soultaker and learned that her husband's spirit resided inside her sword, allowing her to hear his voice.
Originally introduced as a regular member of the superhero team The Outsiders, Katana eventually received a short-lived ongoing comic book series and recently appeared in the pages of Green Arrow during the storyline "The Outsiders War." The character has also appeared in two animated series, Batman: The Brave and the Bold and Beware the Batman.
*** UPDATE ***
The article states that Maseo "will be introduced in flashbacks as Oliver’s handler. Over time, he also will evolve into a teacher and a friend who helps Oliver step ever closer to becoming the superhero he is today." And then, presumably, he'll be killed off with his spirit entering Katana's Soultaker.
Arrow returns to The CW on October 8, 2014 at 8 p.m. EST.