The Terran Reaper

Against the Zerg 

Weak Against : Ultralisk, Baneling, Brood Lord, Roach

OK Against : Hydralisk

Strong Against : Zergling

Against the Protoss 

Weak Against : Colossus, Archon,Stalker

OK Against :  Dark Templar, High Templar, Sentry

Strong Against : Zealot,Probe 

Against the Terran

Weak Against : Banshee, Marauder, Siege Tank, Thor, Viking

OK Against : Ghost, Hellion, Marine

Strong Against : SCV 

The reaper is a new unit introduced by Blizzard in Starcraft 2 with the role of raiding.The strenght of the Reaper lies in his great mobility and harrasing power.Reapers are most used very early in the game against Terran and Protoss players.They can decimate an entire mineral line if microed and used properly and can bring an early win for the player.What makes the reaper a pain for your oponent is the ability to jump on and off cliffs,leading to surprise atacks behind a mineral line against an unprepared enemy.The downside of the reaper is that for all his speed and mobility he dies extremely fast,and must be always on the move to survive .

Upgrades

• Nitro Pack : The only upgrade available to the reaper from the tech lab,the nitro pack gives a significant speed increase to the reaper.Great upgrade for kiting and escaping oponents,especially deadly to protoss zealots without the charge abilty .

Using Reapers Against The Zerg 

Reapers are not a very good ideea against the Zerg.The thing with the zerg is that ,while they are great at killing their workers,zerglings can kill reapers pretty fast especially on creep.The only option against the zerg is to go with a slightly higher number of reapers for your rush and have a pretty good micro ability .

Using Reapers Against The Protoss

Reaper rushes are the nightmare of every protoss player .The only defense against a reaper rush for a protoss is the use of Stalkers,when a stalker is on the field,your rush is over.But untill your enemy gets his first stalker,you basically have free reign over his mineral line,kill as many probes as posibble in that time. If he has zealots in the base you can kite them quite easily while you kill their probes.

Using Reapers Against Terrans

When squaring off against other Terran players the role of your Reapers are very simillar as with the Protoss,altough a little bit harder to kite marines,you can kill a good amount of SCV's with good micro.When you see that he has a marauder ready,fall back.