Little is known of her background.
Mrs. Koibuchi comes across as being a very passive personality. Her husband is an adulterer, but her opinion on this is unknown. She does permit his former mistress and their son to come to dinner the night that the family takes the son in.
At other times she's seen trying to relieve stressful situations between Keiichiro and Kuranosuke. Once asking her husband to calm down, another time informing Kuranosuke that she'd prepared a bath for him. That she did this when he'd only just arrived home did not strike him as unusual.
How she feels about Kuranosuke's crossdressing habits is also unknown, but she clearly does not want it upsetting the atmosphere of the household. Keeping the peace appears to be her main objective.