Hard mode:
Beat all five scenarios on Easy or Normal. Hard mode can now be purchased for 2000 points.

Very Hard mode:
Beat all five scenarios on Hard. Very Hard mode can now be purchased for 3000 points.

Infinite Ammo mode:
Beat all scenarios with 100% completion. Infinite Ammo mode can now be purchased for 50,000 points.

Unlock characters:
After unlocking the following characters, go to the Collection option and choose Costumes. You need to buy them with points.

Bob:
Beat Outbreak level as Mark. Costs 4000.

Danny:
Beat Hellfire level and get Ren's ID Tag. Costs 5000.

Doctors:
Beat Hive level as anyone. Costs 6000.

Fireman:
Beat Hellfire level as anyone. Costs 3500.

Frank:
Beat Decisions, Decisions level on Hard. Costs 8000.

Gill:
Beat Hellfire level and get Charlie's ID tag. Costs 5000.

Greg:
Beat Decisions, Decisions level on Very Hard. Costs 10,000.

Monica:
Beat Below Freezing Point level as anyone. Costs 5000.

Mr. Color:
Get all special items. Costs 100,000.

Nurses:
Beat Hive level on Hard. Costs 5000.

Peter:
Beat Decisions, Decisions level on Hard. Costs 5000.

UBCS:
Beat Decisions, Decisions level as anyone. Costs 8000.

Will:
Beat Outbreak level on Very Hard. Costst 3000.