Check Out This Soleus Exercise Article I Wrote

The soleus is a muscle that is primarily composed of slow twitch fibers and is located on the side of the gastrocnemius. It is weaker than the other calf muscle, but lends itself well to the diamond shape and is relatively easy to work. Rotating between this muscle and the gastrocnemius is the best way to build huge mass in your calves and gain a ton of functional strength.

You can check out my article here soleus exercises where I detail how to build up the soleus muscle the most effective way. Exercises that I explore are the seated calf raise, barbell lunges, and a 45 degree calf extension. All of these are great exercise that'll add some serious mass to your soleus if done correctly.

You may also want to check out these leg strengthening exercises.