heel
跟 centers on the heel of the foot, the back part that strikes the ground when walking.
跟 combines 足 (foot) with 艮, which likely contributes the sound. The character specifically refers to the heel.
The left side 足 means foot, and the right side 艮 looks like a bent leg with a protruding heel. Picture the foot's heel as the part that sticks out like the shape of 艮.
For コン, imagine a con artist kicking you in the heel: con -> コン, and your heel hurts from the kick.