Although you only control your batter in ballgames, how you upgrade your pitchers determines how well your opponent does against you.

To view a pitcher's specific skills, tap the "i" button to the upper left of the player's  portrait:

• ARM: pitcher's ARM rating shows how hard he throws the ball.
• CTL: pitcher's CONTROL rating determines a pitcher's accuracy throwing the ball
• STF:  pitcher's STUFF rating measures how tough his pitches are to hit. In this case you can see the pitcher has several skills (fastball , slider, cutter, changeup, sinker, curve)

In addition, if your pitcher is an X-factor player, you'll see an option to equip an X-Factor skill for your pitcher that can give the player even more value. 

If you're out of X-factors to equip players with, you can always get more through mystery boxes iin the store.