How to Unlock The Assassin
To unlock Assassin, you have to clear one level without killing or alerting anyone. This means that you can chloroform/arrest people, but not kill them.
Choose Shapeshifter (as he is fast and small), then restart until you get “Operate” (flick three switches) and “Retrieve Item” as the missions. Complete the missions, go to the elevator, and you should unlock Assassin.
Choose Thief, as he starts off with 3x Lockpicks, 3x Safe Busters, 3x Window Cutters, 3x Cardboard Boxes and 5x Wall Bypassers, and of course his ability is Sticky Glove which is useful for the “Retrieve Item” mission. You can do any mission apart from the “Neutralise” with Thief. Just complete the missions undetected and without killing anyone, then go to the elevator, and you should’ve unlocked Assassin!
Choose Doctor, as you can complete any missions with him, and he can chloroform people every 10 seconds. Make sure you don’t get detected though, or “kill” anyone.
Use Scientist and tranquilise your enemies. It’s silent, and they don’t feel it. The only downside is it takes ten seconds for the person to fall asleep.