Unlock new officers by doing the following:

Magoichi: Clear any campaign
Masamune: Clear any two campaigns
Shingen: Clear Sanada's campaign
Kunoichi: Clear Hanzo Hattori's and Shingen's stories
Keichi Maeda: Clear Kenshin Uesugi's story
Okuni: Clear Keiji Maeda's story
Goemon: Clear Okuni's story
Ranmaru Mori: Clear Mitsuhide Akechi's story
Noh: Clear Ranmaru Mori's story
Nobunaga Oda: Clear Oichi's, Magoichi's, and Noh's stories