Disclaimer: The "(post reunion)" thing is very important as she pretty much underwent a polar shift in her personality before said reunion happened. Her position in the timeline affects both her personality and her backstory.
Name: Erythema
Gender: Female
Species: Donkey
Age: mid 20s
Appearance: From the neck-up. Her entire color scheme is exactly like her twin brother's
Quick facts: A foreigner, lives with her twin brother in Fillydelphia. She and her brother work together and do odd jobs.
History/Background: She's from an island far off of Equestria's shores, one that is mired in territorial wars between different clans. Key events in her life gave her the conviction to become a skilled warrior where she has become feared by friends as well as foes. Her ideal became to protect not only herself but those she holds dear. With her parents now deceased, her twin brother is the only family she has left and she has made a promise that he won't die before she does.
Some time ago she insisted on accompanying her brother and the rest of the crew on a mission to discover potential new islands to migrate to. However, they got shipwrecked and washed up on Equestria's shores where everyone got separated. Her new conviction had become to find her brother and go back home. Because she had been very aversive to interacting with the locals, that went very slowly. Another important thing that was happening were her observations of Equestrian societies but in her stubborn refusal to let her guard down, all the clues just kept floating in the back of her mind. She was eventually found by a search party and got reunited with her brother. As the reunion happened, she heard of the kindness of the locals as well as having seen it in this reunion. All the pieces fit together now and it finally sinks in that her cold, barbed steel exterior has lost its relevance, at which point she had a breakdown. For days in the aftermath she was unable to speak and she clung to her brother like a scared filly to a parent. Now she is left wondering how she's going to live in a world that seems so utterly different than what she grew up in.
Personality: Quite shy and somewhat easily frightened. She prefers to stay by her brother's side but she's still having a hard time feeling safe on her own (and unarmed). In most interactive situations, she would rather let her brother handle everything -- again turning to him as though he were a parent. She just comes off as very confused, indecisive and overly alert of her surroundings. It's as though she never really knows what to say, how to talk or how to act and she's too concerned with what might go wrong. Needless to say at this point, she'd be very awkward to interact with.
Added: As time passes and her more vulnerable side is coming out, slowly the sins of her previous life as a killing machine is starting to catch up with her. As her vision on the front lines of war continue to fade and blur but with her memories still vivid, she is unprepared to cope with these memories as her vulnerable self.