Diamond no Ace OVA

Diamond No Ace OVA covers the backstory of the Kawakami, Kuramochi and the Kominato brothers. It's really nothing special at all. It's just three filler episodes. The first episode covers the Kominato brothers and pretty much retells the same story as you already got in the main series, about Haruichi following in Ryousuke's footsteps. There's literally nothing new here except a little extra footage showing how Ryousuke ended up at Seidou.

The second episode is more entertaining, as it follows Kuramochi's path from a delinquent who was rejected by all the high schools he wanted to attend because of his ... behavior, to being chosen by Rei to join Seidou. This was the funniest of the episodes.

The third episode was kind of a snoozer like the first episode, as it shows Kawakami struggling to believe in himself and Miyuki, Azuma and some of the management and staff helping him to believe in himself. I've never been a huge fan of Kawakami, as he's more of a "Yamcha" character to me, so I wasn't really all that interested. Besides, Kawakami's character arc in the main anime is still pretty much the same. A guy with decent talent, a fourth-stringer, struggling to believe he belongs. But, let's be real here, the guy is a supporting character playing the same position as the two main characters in Sawamura and Furuya. He's pretty bland, nothing much stands out about him, and he's easily overshadowed. He's Yamcha behind Goku and Vegeta. He's Kiba behind Naruto and Sasuke. He barely matters, which is evidenced by the fact that most of the time he pitches in the main series, the episode just skips past it and let's you know he closed out okay. Even they know you don't care. But, he got his own episode in this, so that pretty much lets you know how crucial this filler is.

You don't need to watch it and you won't miss anything if you don't. It's not bad, but it's just some mailed-in flashback episodes.