Marsti is an indifferent and apathetic janitor, not particularly concerned with anything or what people think of her. She appears to have a hard time giving a damn about other people, and she blandly asks why the player feels the urge to hurl themselves into dangerous situations. She also snaps that she couldn't care less about other people's opinions of her, even though she has a tendency to become rather defensive when people question her motivation or occupation. Her snarky and cynical demeanour can be attributed to the fact that she's at the bottom of the Alternian social scale, since it means that she can never really amount to anything by virtue of the hemospectrum. 

Marsti is also shown to be a private person, outright stating that she isn't about to tell her whole life story to some random alien on the street. When the player attempts to have a conversation with her, Marsti isn't impressed with the player's desire to socialize, claiming that they're doing a piss-poor job of being a lookout as they're too busy asking her invasive questions. Even in the few instances where she lets the player in on a bit of info, the details are minimal and brief.

Apart from that, Marsti is somewhat irritable and impatient. She finds the player's presence exasperating at best, their remarks and comments annoying and poorly-worded. When the player inadvertently implied that she's not enthusiastic about her current job, Marsti doubles down on her work just to prove that the player's wrong. She also thinks answering the player's numerous questions to be tedious in nature, for the player sometimes gets confused with troll terminology and cultural differences. Should the player keep making references to friends made in the past, Marsti grumbles that not all lowbloods are acquainted with one another, clearly getting frustrated with the player. As such, it can be inferred that she's slightly abrasive in terms of personality, though she does have a soft side to her. 

Despite her gruff attitude, Marsti is still capable of caring for others. Upon meeting Folykl, Marsti quickly notes that the goldblood has voidrot and offers advice on how to manage her ailment better. Marsti displays a surprising amount of knowledge regarding biology and medicine, even though she's fully aware that there is minimal chance that she would be a medic off-planet. When Galekh's goat lusus tries to eat her glove, she merely scratches the goat behind the ears as a distraction so that she can gently extricate her hand, hinting that she's not one for constant confrontations.