Some units have alternate method of unlocking

Aestivalis Akatsuki Clear extra mission 3
Aestivalis C (Hikaru) Clear 15 secrets
Aestivalis C (Izumi) Clear campaign mode
Aestivalis C (Ryoko) Clear 20 secrets
Alstromeria Clear extra mission 2, or recieve 30 ace rank
Auge Clear 60 secrets

Bilbine Camo Color Upgrade Bilbine's special stat enough to evolve it
Black Selena Clear campaign mode

Daljian Clear 25 secrets

Gilgazamune Clear 35 secrets
GP02 Clear extra mission 2

Johnathan Baronz Clear 60 secrets, or clear extra mission 1

Layzner MkII Cocept version Upgrade Layzner MkII's special stat enough to evolve it

Nanajin Available in Free mode
Nelly Brain Upgrade Yuu Brain's special stat enough to evolve it
Neue Ziel Recieve all ace rank

Oukaou Clear campaign mode

Quincy Baronz Clear 50 secrets

Yatenkou Recieve 60 ace rank

Zakarl Clear 40 secrets
Zwarth Clear 30 secrets