Here’s another confirmation from Terajima-sensei that “Ace of Diamond” is centered around the relationship between Miyuki and Sawamura.
So this is the special interview with ‘Ace of Diamond’ author Terajima Yuuji and ‘DAYS’ author Yasuda Tsuyoshi (source).
―What kind of things did you keep in mind when you created characters like Sawamura and Tsukushi (MC of ‘Days’)?
Terajima: “I had planned to create a story where the main character strikes out batters with his moving balls, but the thing that was most important to me was the relationship between Sawamura and Miyuki.I wanted the story to center around Sawamura’s feeling he had when he met Miyuki: “I want to play baseball and form a battery
with him”. So the starting point of Sawamura’s character wasn’t really his play (way of playing), but his relationship (with Miyuki).” […]