Tatsumi's parents died in a fire that the Chief Fireman couldn't save them from. Wracked with guilt, he took her in and raised her as his own daughter. She's dead set on following in his footsteps and becoming a firefighter herself, though he's pretty against the whole idea.
When a string of arson attacks hit Edo, she got a little too eager to catch the culprit and ended up staking out the area from inside trash bins. This is how she famously mistook Gintoki for the arsonist and beat him up before realizing her mistake.
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