Air carrier First Officer reported the flight crew loaded the wrong fix and flew approximately 8 miles off course before being corrected by ATC. The reporter stated that the fixes KP12E and KP21E are quite similar and also close in proximity; which led to the mistake going unnoticed during route verification.

Narrative

The Captain and I set the Multipurpose Control Display Unit (MCDU) route and verified fixes per procedures during preflight. However; one of the assigned fixes along our cleared route was loaded incorrectly. KP21E was the fix we were cleared to. The Captain and I incorrectly entered the fix KP12E and did not catch our mistake during our preflight checks and MCDU verification. This caused us to deviate approximately 8 miles off filed course before ATC corrected us.The cause of this event was a combination of human factors and the fact that those fixes are so similar despite being close together. KP12E and KP21E are only 90 miles apart and very similar; which caused the mistake to go undetected when checking the plan view of the route as well as reading and verifying the PDC. The Captain and I believed we had the correct fix loaded. If the fixes were not nearly identical we would have recognized it was not the correct fix.Perhaps two fixes in such close proximity should not be so similar; and crews should take extra care in route verification especially when there are fixes with numbers.