The undisputed boss and arbiter of justice in the camp, everything would have fallen apart years ago without Susan in charge. Tenacious and iron‐willed, she stands for no nonsense.
—Rockstar Games' description of Susan.
Susan was one of the original members of the Van der Linde gang, alongside Dutch van der Linde, Hosea Matthews and Arthur Morgan. She became romantically involved with Dutch at some stage, and long nights playing poker with the gang, often sitting in Dutch's lap. Their relationship later ended, and Dutch eventually moved on to another woman named Annabelle. Despite that, she remained a close associate to Dutch, remaining on good terms with him and gaining his respect, eventually becoming the gang's arbiter.
By 1899, alongside with the gang's cook Simon Pearson, Susan was in charge of setting up the camp every time the gang moved to a new place.
When the gang settle at Colter, she organizes the camp and decides where each gang member’s rooms will be, prompting complaining from Micah after he is assigned to a shared room with Javier, Charles, and Bill.
Grimshaw shows Arthur his living space, and tells him that all of his stuff from Blackwater was saved. Later on, she informs Arthur that a letter from Mary arrived for him, and expresses disdain for her.
Grimshaw asks Arthur to help investigate Tilly's disappearance, who she believes was kidnapped by the Foreman Brothers. The two ride up to a Foreman Brothers hide out. The two storm the safe house and rescue Tilly, before realizing that the three Foremen Brothers have fled. Arthur chases after them, and shoots two of them off their horses before capturing Anthony Foreman and bringing him to Tilly and Susan. Susan will encourage Arthur to execute him, although it is ultimately the player’s choice whether to kill Foreman, or spare him.
When Molly comes into the camp drunk, repeatedly insulting Dutch and stating that she told the Pinkertons about the bank robbery in Saint Denis, Grimshaw shoots Molly with a shotgun, saying ”she knew the rules”, and subsequently ordering the body to be burned.
Later on, during the gang standoff at camp after Micah is revealed to be the rat, Susan points a shotgun at Micah, demanding that he puts his guns down. Suddenly, Javier announces that Pinkertons are coming, which distracts Susan. Seeing his chance, Micah shoots Susan in the stomach while she is off guard, and she quickly succumbs to her wound. Miss Grimshaw is later buried by Charles Smith. Her grave can be found in Roanoke Ridge, southwest of the Elysian Pool, on a cliff nearby.
Miss Grimshaw is heavily depicted as a forthright, "strict maternal figure" towards the Van der Linde gang. She admitted to Arthur that some people need a firmer hand than others, whist adding that someone has to keep the rabble in line, otherwise they would "rot in their own filth".
Despite having a tough love attitude towards the gang's members, she does care about them, as evident where she had Arthur assist her in rescuing Tilly Jackson. In Beaver Hollow, she privately told Tilly that she has grown into a "fine young woman".
In spite of her maternal nature, Miss Grimshaw displayed a ruthlessness towards enemies, whoever they were. She did not hesitate to kill Molly after her confession of talking to the Pinkertons, and threatened Micah at gunpoint after discovering his betrayal.
Sometimes, when Arthur is in camp, Susan may approach and comment on his recent actions. Activities she will discuss include killing innocent civilians, avoiding donating to the box, and spending too much time at camp, among other examples.
If the player does not have a bath for a long enough period of time, Susan will forcibly wash Arthur in a water barrel in camp.
She is physically abusive towards Mary-Beth, and can be seen slapping her face in camp for not doing her chores. It is reasonable to assume that she is physically abusive towards the other young women in camp as well, due to the disdain they have for her. Karen, in particular, expresses a vehement dislike of her, and may crack jokes with Arthur about it, and Tilly will ask him if Dutch "would be mad" if she killed her.
Arthur says that Dutch has a "soft spot" for her.
During the Beaver Hollow chapter, Susan may approach Arthur in camp to tell him that someone smashed the donation box, thus explaining its absence.
She repeatedly mentions her dislike of Mary Linton, much to Arthur's annoyance.
Out of all the women in the camp, Ms. Grimshaw has little to no interaction with Sadie Adler.
Ms. Grimshaw appears to favor shotguns, as that's the weapon she wields most often in cutscenes.
Similar to Simon Pearson, Susan is mostly addressed by her surname.
Grimshaw is described in the credits as a "retired painted lady". The term "painted lady" is an archaic word for a prostitute, indicating that this was Susan's profession prior to joining the Van der Linde gang.
She seems to somewhat dislike Pearson's cooking, which is the reason she asks Arthur to find oregano for seasoning.