Is there a relationship between an increase in kenitic energy with a decrease in potential energy?

Yes, there is a relationship.
If the pendulum was held at a height h, it would have only gravitation potential energy (GPE), which can be calculated by mutiplying it's mass, by the height h, and g, the gravitational pull (for Earth, we can assume this is a constant 9.81m/s2).
When the pendulum is released, it gains kinetic energy. But due to the law of the conservation of energy, we cannot gain or lose energy, therefore there is a reduction in the GPE, equal to the kinetic energy, at a time t. (mgh+0.5mv2 = 0) Once the pendulum reaches it's maximum velocity, there is no GPE for a fraction of a second. It then gains GPE again asit works against the gravitational pull. This can be proved with the equations:
-> GPE = mgh (m = mass, g = acceleration due to gravity, h = given height)
-> KE = 0.5mv2[squared] (m = mass, v = velocity)
the mass cancels out on both sides
gh = 0.5v2
Which implies => 2(gh) = v2
Which implies => square root of 2(gh) = v
I hope this helps you in any way. o/