Willow is a buffalo companion here to help you reach your water goals on certain levels.   Once he's received enough (triangular) companion dots from you, he will add drops of water as well as Blank Dots on your board. Once he's added water to the board, make a connection next to the water to spread it.

Fill up Willow's meter with five companion dots, and his ability will be activated. (With a boost, his meter can be filled with only four companion dots.)