KUALA LUMPUR—With her pink diamonds, brightly colored silk robes and vast collection of Hermès Birkin handbags, former Malaysian first lady Rosmah Mansor for years was a lightning rod for anger over suspected corruption. Cartoonists portrayed her in the mold of Imelda Marcos or Grace Mugabe: a big-spending spouse whose husband’s position kept her in luxury goods.

Now a sharper portrait of the fiery 66-year-old Ms. Rosmah is emerging from interviews with investigators and people close to her, one that paints the wife of former...