Is the first half
And the latter half?
My counter has a speed,
And loves to play with fire.
The opposite of darkness is light, and it travels at the speed of light. Light is an integral part of what makes fire
The set becomes my seed,
When twinkles then respire.
I believe this is referring to the sunset. Twinkles typically refer to stars. So when stars respire or set, darkness comes.
I am not transparent,
My leading property.
Darkness's leading property is that it's opaque or black. The point of darkness is that you can't see clearly.
I might not look so pleasant;
The more, the less you see.
Darkness can be scary, especially because you can't see anything. The more darkness there is, the less you can see.
Is the latter half of the riddle referring to
My friend is also yours;
It follows all the time.
Shadows are attached to you and are ever present.
The one rule it ensures,
Is that you are its prime.
Shadows always originate at the person's feet, hence it's prime.
Attached, it never leaves;
It holds on tight 'til end.
Again, shadows are attached to people.
I am dressed by thieves,
When crimes will impend.
Crimes are often committed in the dark (shadows), and shadows are used by criminals to hide their misdeeds.
The first half is about myself, and the second half is about my friend, even though our friendship is not very light.
Both are pretty much the opposite of light, but are compliments (friends) to each other.
Title: Your Friend Will Hide Away, If Light Will Not Display
The friend here, as mentioned, is a shadow. If there is no light, there can be no shadow either, only darkness.