Super Robot Wars/L/Secrets

Secret	Conditions
Duel Gundam Assault Shroud Buster Gundam	On Scenario 6 protect and secure all 6 Meteor Breakers. You will then unlock them after Scenario 33.
Rey w/ Legend Gundam	On Scenario 33, use Kira to have a battle encounter with Rey, then have Shinn shoot him down. He will rejoin in the next intermission.
Stellar	On Scenario 15A, use Shinn to reduce the Destroy to 10% HP or less Get her around Scenario 32.
Atross	During Scenario 23, use Iczer-3 to defeat her after the 50% HP event. Scenario 26, use Iczer-3 to shoot her down. She will survive after Scenario 34

The following secrets are not fully confirmed yet:

Secret	Conditions
Michael	After Scenario 28, take the Space Route. Have Michael kill at least 8 enemies during Scenario 29-A. He will remain in your team and survive.
Soubi w/ Talisman	During Scenario 12, have Hayase shoot down Soubi. After Scenario 14, chose the JUDA path. Scenario 15B, have Linebarrel fight with Soubi. After Scenario 18, pick the Earth route. Scenario 31B, Fight Soubi twice with Kouichi, then shoot him down with Kouichi. Scenario 33, Persuade Soubi with Kouichi. Scenario 37, Persuade Soubi with Sawatari, then shoot him down with Sawatari.
Ken w/ Blade Gainer	The mechanism by which this secret can be unlocked is still unclear, as different people have reported different paths to recruiting Ken. Some or all of the following may be a factor: After Scenario 19, take the Minerva route. Before Ken and Lou appear, shoot down the Memetic Beast twice. Have Godannar (TDM) kill the boss Memetic Beast. Destroy all enemies before finishing the boss.

Source