This religion worships Yggdrasil, the World Tree. His location is unknown and rumored to be in a spiritual dimension rather than an actual one. But some highly skilled Grassmen can make themselves in tune with his esscence and receive messages.
Because he generally doesn't appear, it is unknown how large or what type of tree he resembles. The few times he appeared in a vision, he would take the form of a Grassman. However, it is widely known that isn't his real appearance and that he really is a tree. Rumor also has it that when he dropped two twigs one day, he willed them to become godesses, birthing Natura and Malia.
Because he is simply a tree who can take the image of a Grassman, he really hasn't had much of a role on history. After the Grassman Clan was created, he simply bowed out through the prehistoric wars and the Ice Age. To him, it is important for people to find their own knowledge and face whatever hardships might appear.
Grassmen don't live in villages like the other clans and travel to seek knowledge once reaching adulthood. So the ways they worship Yggdrasil are unknown. Most of what is written is that their quests for knowledge are their form of worship as he is a wise being.
When Grassmen are several hundreds of years old, and have amassed enough knowledge, they will plant their roots and grow to a Wise Tree. These trees are connected to Yggdrasil by the roots, or so the legend goes. Thus, even a non-spiritual member of their clan can now get messages and insight from their deity. At the behest of Yggdrasil, they now impart knowledge to others when it is needed, whether or not they are Grassmen.