Grace Monroe has BPD
A little background on Grace. In Book 3 it is explicitly confirmed that her parents emotionally neglected her. They never showed up to any of her recitals and always pushed her away in favor of their work as diplomats. They kept her out of site and isolated away from most kids and was unable to form meaningful connections with kids her age, nor with any adults in her life. So when she arrived on the Infinity Train, she latched onto the first availible "authority figure" that "saved" her. I aim to argue that Grace Monroe has BPD. More Specifically, impulsive and petulant BPD
✪ Grace repeatedly tries to avoid abandonment/rejection, at times doing really reckless/impulsive things to keep this from happening in her personal relationships (this fails).
As stated in "The New Apex", Grace is afraid of being alone and not being enough for other people, so she does whatever possible to keep people from leaving her in the way her parents essentially abandoned her. For years, Simon has been the only person she trusted with her life. The writers and creators of the show consider her relationship wth Simon to be co-dependent, indicating that it was founded on unstable grounds and grew omre unhealthy as the two grew up.
Grace is shown to value her relationship with Simon above that of others, being that she repeatedly hides Hazel's true nature from Simon to keep him from getting upset with her or harming Hazel.When Hazel is finally exposed, Grace ends up throwing her under the bus to save face with Simon. This doesn't even matter in the end because Simon found out that Grace knew all along and their relationship is irrevocably damaged.
Even by the end of the season Grace is still trying to talk down Simon and try to fix her mistakes while Simon is trying to kill her. She still had hope that she would be able to get Simon to turn around and make things better. But things can never go back to normal between them.
✪ Grace displays a lack of "self" or "identity".
Grace's identity, her actions and her morals up until the middle part of book 3 revolve around the idea of the "true conductor" that "saved" her life when she was an impressionable child. She is the leader of the Apex, a band of rowdy kids dedicated to honoring the "true conductor" by using the train as they pleased. But once her perception of the "true" and "false" conductor falls through, it is shown that Grace doesn't understand much about herself beyond what the "true conductor" meant to her. And now she'll spend the time after book 3 trying to figure that out while getting herself and the remainder of the Apex off of the train.
✪ Grace engages in dangerous, life-threathening activities
Being the Apex, Grace performs a lot of stunts and feats while causing destruction in the train cars. Sometimes this gets her or others hurt.
✪ Grace has a favorite person
This may or may not come as a surprise to some, but I believe Grace's "favorite person" is not Simon, but in fact the "true conductor". She holds the conductor on a high pedestal as someone that every passenger should look up to. She and every member of the Apex wears a lipstick sine wave on their face to replicate the waves seen on the machine the conductor came out of. Furthermore, as the person with the presumably highest number, she feels a sense of duty to encourage others to get higher numbers to honor "him". If she is unable to live up to the expectations of the true conductor, she feels weak and worthless to the Apex, especially Simon.
✪ Grace experiences memory disturbances
In Grace's tape, there are inconsitencies displayed in her memories that are corrected once Grace starts to remember things as they actually happened and not what she believed happened for so long.
Now, onto why I believe she specifically has petulant and impulsive BPD. People with petulant BPD feel a need to manipulate or control others, which Grace is very explicitly confirmed to have been doing with Simon, the Apex, and Hazel. She admits to this in her conscience. She is even shown riling up a group of girls in her childhood to fight with each other as payback for excluding her. Additionally, people with petulant BPD are prone to mood swings, defiance, irritability, and passive-aggressive behavior, all of which Grace increasingly displays towards Simon and even Hazel as book 3 progresses. As for the impulsive subtype, I've already explained her risky behaviors engaging with the Apex, so I think that speaks for itself.