From Blaseball Wiki

Sprout is the batboy for the Boston Flowers. He is an orange tabby cat dressed in a papier-mȃché tree costume.

To the rest of the team, Sprout appears to be yet another sentient (albeit stubby) sapling that happens to be very good at carrying blaseball bats around for the team. As a kitten that lacks both grace and opposable thumbs, Sprout is unable to take off the costume to reveal his deception, however unintentional and adorable it may be. In light of this, he has found that sleeping on the job is encouraged.

Members of the Flowers that are most suspicious of Sprout include Alaynabella Hollywood and Glabe Moon; most of the team has attempted to hold interventions for Hollywood in order to convince her that Sprout is, indeed, just a sapling that happens to meow and likes to chase after mice in the Garden, while Moon is still confounded by the difference between flora and fauna.

It is a mystery who put Sprout into the costume in the first place.