He brought ILMATAR into being. He specializes in Storms and Lightning.
He’s also the source of a mysterious celestial beverage which drops from the skies. This is not so much manna from heaven as honey from the clouds. It’s not only sweet but also has amazing adhesive properties (see LEMMINKAINEN).
If you need more info, you could be out of luck. UKKO is somewhat aloof. There’s no point in trying to reach him unless all other prayers remain unanswered. You could try his wife AKKA, who as Earth Mother Goddess is far more down-to-earth.
Once the Top God of Finland, UKKO found his popularity reduce as Christianity spread through the country. See JUMALA.