Sierra is 5'7" with long pinkish/purple hair which she usually keeps tied up in ponytails of various types and sparkling blue-green eyes. Typically, she's wearing very loose tops and very tight pants or shorts, thus granting her the nickname of Bursting Breaker, because her tops seem to always become undone. She's incredibly fit as she's almost constantly training for her next fight, however, despite her toned physique, she has incredibly large breasts that seem to easily distract those of a more perverted nature, including her former commanding officer S. Anji.
When on duty she wears a standard marine coat, a brightly colored blouse, a pair of yoga pants with trainers. She almost always wears some form of covering for her hands due to her being an incredibly hard puncher.
Sierra is a rather excitable and energetic person, seeming to always be acting at 110%. She takes justice very seriously, but not the Marine's Absolute Justice. She just simply believes in helping good people and taking down bad ones. Unlike most marines, she doesn't believe all pirates are inherently evil. People have to do bad things to be a bad people. Because of this belief, she will sometimes let pirates who don't seem evil or have bounties for nefarious reasons escape.
When not in marine mode, Sierra can be seen as somewhat simplistic never focusing too much or speaking about any subject in a more in-depth manner. This doesn't mean she's stupid by any means, possessing a decent mind and a somewhat surprising about of perception. She's just simply not an expert on most topics she discusses or simply has little need to demonstrate her intellect. Romance mostly eludes or she may be simply trying to ignore the more 'horn-dogged' individuals or comments that are pointed towards her.
She's aware of her body and how 'attractive' she may appear to others. However, she just refuses to let that be what dictates where she goes in life. She's routinely exposed to due circumstance or her rather ill-mannered teammates, however, she's almost never embarrassed by these experiences. However, this doesn't mean she won't cover up with the first chance she gets.
Sierra grew up in a town in the North Blue that was home to a Marine base. Her parents ran a small Martial Arts dojo, where she grew up training. While her parents wanted her to help run the dojo, but she had always been interested in the Marines and Justice. As she got older her dream of joining the Marines and going to sea to fight for Justice only grew stronger, so her parents finally let her enlist.
Throughout her time in the East Blue, she comes across various scenarios and instances that have caused her to rise through the ranks of the marines and to solidify her beliefs . While sailing in the Grand Line with her Captain she was told about the state of her home region. Though it pained her to leave her unit behind, she personally requested to be allowed a transfer to the North Blue in order to properly help the region. She was apparently granted it for one reason or another, along with allowing her to become a Captain herself. Perhaps someone higher up vouched for her experience.
Sierra's combat style can be summed up in one word. Strong. She doesn't have gimmicks or strategies or thinks too hard while she fights. She simply charges towards the enemy and beats them down as hard as she can. Her fighting style consists of the most basic of punches and kicks, however, due to her years of experience and home training, you can tell everything from her stance to the way she moves is that of a trained professional.
Her stamina is equally as ridiculously high being able to take a surprising amount of damage with little cuts or bruises. Usually, however, her clothes seem to take far more damage than she does. In those cases it's incredibly likely that her large chest will be exposed, usually giving her the advantage to take out pirates while they're distracted.