Stat block is up yonder so...
Onis aren't an inherently violent race mind, but they do like their booze, their ball games and like any proper hooligan a bit of rough and tumble every so often - just so long as nobody gets really hurt. Given that most Onis can regenerate severed arms and legs, 'really hurt' is a level incompatible with most other species. They're well aware of their sturdiness compared to other races and tend to treat them with kid gloves; it's not their fault they're so squishy, and most acts of (to most other races) horrific violence are just taking the piss out of each other and grudges or revenge are rarely taken up.. The highest mark of honor and respect is given to individuals of other races who can beat, or even stand up to, another Oni in matters of physical strength, endurance, and sheer stubbornness.
When they aren't being boisterously drunk, Onis tend to be rather lazy and laid back. Something will occasionally be done about a problem, but that problem's going to be bloody well ignored and worked around until an Oni finally gets the gumption to actually try and fix it. When a problem does get fixed though, it's going to be as fixed as that oni can personally make it - there's no half-assing something an oni decides is going to get done, and that applies to fixing holes in walls as much as it does fixing them. Onis are encouraged to dabble in various arts and crafts and fixer upper activities as a result; hopefully something sticks and an the village will have someone who can fix just about anything if they're given enough incentive to actually care about fixing it in the first place.
Marie was encouraged to be a law enforcer at a young age. Law enforcement in oni villages largely comes down to 'motivating' other Onis to get something done at times, and requires as much an ability to kick heads in as the ability to make it hurt in the long term as well. Well connected to the bureaucracy and used to working in groups, Oni Law Enforcers were the physical boot to the bureaucrat relentless grind. She took well to the task, being more motivated than the average Oni and being able emotionally stable and patient enough to handle even the most reluctant Oni. As well as giving a stronger than usual good swift kick to the ass when needed.
When the human/monster girl cultural exchange program was established, high level politicians realized the need for monster girl staffed security to prevent monster girl on human violence. Most monster girl on human crimes gave monster girls a great advantage short of humans resorting to lethal force, and that was easily seen as a potential for diplomatic nightmares.
Marie was a good candidate for the position. Trained in security and body guard issues, laid back enough to handle the shenanigans expected from some of the more sexually open and aggressive monster girls without getting tweaked out by it, as well as having a hell of a heavy punch when it came down to it, Marie quickly became an asset for M.O.N. They don't mind her moonlighting on the side either, more 'personal' relationships between MON members and humans are encouraged as long as it doesn't interfere with the job.
She's acquired something of a reputation as being clean and discrete for hired muscle. She doesn't mind those parties with the fancy dressed servants serving wine and tasty cheese samples either, when she can get to them, and is avidly learning what she can to fit in better to all levels of human society from gutter trash to hoity-toity. This is her calling in life after all, and it's going to be done right.