Eliza

Eliza (エライザ) is one of the last 4 Thieves' guild mentors and seems to act as its de facto leader since Barbara's death.

Eliza is elusive and most have never seen her nor have they hear her voice. Even when she does show up, she still hides most of her body and, sometimes, even modifies her voice. She mostly acts that way because of how shy she is, something both her colleague and herself are fully aware of.

Appearance
While Eliza usually stays hidden, even when talking with people she knows, the situation after the fall of Alterna has forced her to show herself more.

According to Haruhiro, she is a petite woman with long hair which almost completely covers her eyes. She wears a dark-colored outfit and a scarf which hide most of her body, making it hard to make out her figure, and her hands are covered by cloves, leaving only the tips of her fingers exposed.

Matching her appearance, Eliza's voice sounds feminine but she is apparently able to easily alter it.

Personality
Eliza's most notable trait is her extreme shyness. She doesn't want to interact with people and dislikes being seen, especially her face. That shyness seems to be the reason behind many of her life choices: like becoming a mentor or the way she looks.

Other than that she seems surprisingly normal, albeit a bit cold/professional.

Level. 15
After Barbara lead Haruhiro and the rest of the recon team ￼into the thieves guild hideout, she used the excuse of a one on one meeting with him to introduce Eliza who was hiding deeper within the hideout.

After sharing some information, it's decided that Haruhiro will be made into a mentor forcing Eliza to reveal herself for the ceremony.

Then, while Barbara goes Back with the recon team, Eliza stays behind to gather intelligence and maintain contact with the other 2 mentors working outside Alterna.

Level. 16
During volume 16's events, Eliza mostly continues to gather intelligence and act as a link with the volunteer soldiers. She does accept to help Haruhiro to look for Shihoru but it doesn't seem like she managed to get any useful info.

Abilities
Eliza's abilities are still mostly unknown but it seems like the focus more on intelligence gathering and manipulation, than combat. Still, since she was able to become a mentor for the thieves' guild, she probably is quite the skilled fighter.

Barbara
Barbara seems to have been the closest thing Eliza had to a friend. The 2 had become Volunteer Soldiers almost at the same time (Eliza was from a slightly younger generation) and later both chose to leave to become mentors in the thieves' guild. Because of how shy she is Eliza would speak to almost no one and if she had to she would keep it short, but Barbara was an exception as she would sometimes speak normally to her and even admit (to herself) that she liked that kind of conversation.

Haruhiro
While they barely know each other, it seems like they do somewhat trust each other, likely because they're both Barbara's acquaintances and both among the last member of the thief guild.