He’s only there to help her out and act as a friend and mentor.
Supergirl’s writers and producers wanted to make sure that Supergirl was well established in her own right before her more famous cousin showed up and stole some of her thunder.
The show made it clear, once it was finally announced that Superman would, indeed, show up on Supergirl, that the intention was never to have him be the focus of the series when he appeared.
In fact, we’ve already been told that this was one of the reasons that the character didn’t come around in any real way during Season 1.