During the post game section, you can unlock additional character classes. Go to the green SOS signal on your map and rescue the castaway. Teleport to Transporter 04 in the FOB map and search northeast in the jungle to recover the castaway. On the path to the castaway, you will encounter new enemies, such as burning Wanderers that cannot be killed. You will also encounter armored Wanderers that are difficult to kill. Do not try to fight them yet, as it is easier to just run by them. To unlock the new classes, access the final memory board located in a small village north of Ruins 03 in the FOB map. Use the Walker Gear marker located at Ruins 03 to make it easier. The following are the bonus classes:

Assault: Close-quarters combat specialist.
Jaeger: Marskman specialist.
Medic: First aid and support specialist.
Scout: Infiltration and maneavers specialist.

Note: You can only set one sub class at a time. After choosing a class, set the skills in the sub class loadout, as you will not be able to use all skills in a sub class at the same time. The only skills that do not need to be added to the loadout are the stat boosts.