The Ghost skill tree is for players who want to keep moving - get in, grab the cash, get out. Skills here are all about movement. Good ones to unlock include...
...Sprinter. This is a solid upgrade to your stamina, lessening its cool-down time and quickening its regeneration speed. It gets you moving faster, quicker. Does that make sense...?
...Cleaner. This allows you to deal additional damage to special enemies. This skill tree is the least damage-dealing, so any damage boosters you can get - especially against tougher enemies - is helpful.
...Chameleon. With this skill, civilians are less suspicious of you. It's handy for setting up before springing into heavy action.
...Shinobi. This increases your all-around movement speed. Doesn't need much explanation, eh?
...ECM Specialist. This gives you a second ECM jammer. Just like the Doctor Bag and Ammo Bag, having an extra tool will always come in handy.
...ECM Overdrive. This will extend the ECM jammer's duration - that's handy. The big thing here is acing this skill. Doing so will upgrade the ECM jammer to be able to open certain keycard doors.