What kind of friend has Uno become to Danny?

The development of the friendship between Danny and the other kids is a theme throughout the book. Although Danny feels like an outsider much of the time, he does gradually get to know the others, and he becomes friends with some people who initially did not make him feel welcome. His participation in baseball both advances and impedes some of those changes. His friendship with Uno is among those we follow through the book.
Baseball initially is a wedge driven between Uno and Danny. At first, Uno resents Danny for being better at baseball, and he hits Danny, using the excuse that Danny hit his brother. Over time, however, baseball brings the two together. One day while Danny is starting to work out, Uno interrupts him with a story about why he quit organized baseball. Danny realizes that it took a lot for Uno to confide in him.
Later, Uno helps Danny with baseball through practicing with him. Danny gradually comes to realize his pitching talent might be something special. Uno acts as his catcher. At the point where they are going to separate, as Danny is returning home, Uno

wonders if the kid’s tired of this setup. Throwing pitches to a regular old catcher. He’s known all along that this is a temporary thing, that Danny would eventually move on to bigger and better things.

Uno is surprised to find that saying goodbye is going to be hard; that idea “[m]akes him feel kind of soft.”
Having a friend in Uno, as the team faces the rival Leucadias, helps Danny feel like a part of his new community and understand that his own pitching is just one component of his personality.
https://books.google.com/books/about/Mexican_WhiteBoy.html?id=XvCIO26-TxwC