(Mystic Code Book Chapter 1 Character Profiles | Chapter 99)
You already highlighted her cutting her hair in order to add further authenticity to the fake corpse the kids burn for the escape.
There's a lot of significance placed upon maintaining long hair in a number of cultures, including Japan, which is why even though Shirai purposefully chose to base the series in a Western setting, the influence of Japanese culture contributes to the story pausing and reflecting on the weight of that act twice.
(Mystic Code Book Chapter 1 | "Why Is It Such A Big Deal When Anime Characters Cut Their Hair?" by Justin Sevakis | "Of hair and hairdressers in historic Japan" by Debra Daley | Chapter 34 | Chapter 38)
Her other big moment comes roughly a hundred chapters after being given small earlier nods: going with Emma, Ray, and Hayato to retrieve the medicine for Chris. Similar to giving up her braids, she steps up to save her younger brother without a second thought.
(Chapter 50 | Chapter 59 | Chapter 116)
Lucas, Sandy, and Zack all recognized her prowess in the field of medicine (and she's so adorably embarrassed by all the praise with the full covering of her eyes and blush. |3) and with the other three kids watching her back at the farm, they fully believed she was capable of bringing back the correct medicine that would save Chris' life.
While one's childhood aspirations and inclinations don't necessarily last into adulthood (we see Sandy opt for something related to entertainment in the human world after being one of the Goldy Pond Resistance's medics for years), I headcanon that she continued on this path to become a doctor with the talent she has for it.
(Other in-universe post-canon memes here)
Another thing I love are the little scenes where Anna and Nat are included in that older group of escapees to plan their next moves that will simultaneous get them closer to achieving their ultimate goal of reaching the human world while keeping their younger siblings safe and as worry-free as possible. It's another testament to their emotional maturity, mental fortitude, and how much they care about their family.
(Chapter 48 | Chapter 56)
People also tend to focus on Ray and Jemima's bond for understandable reasons, but it's super sweet how often Jemima gravitates toward her older sister. She's probably been there to comfort her since Jemima was first assigned to the same bedroom as her at Grace Field once she aged out of Isabella's room.
