What forces (people, places, ideas) play a role in shaping Siddhartha's journey?
As the song goes, "Three is a Magic Number" in religion, literature, and philosophy--and for good reason.
Siddhartha can be divided into three parts:
In part 1 (chapters 1-4), Siddhartha could have gone to school, been a student and pampered member of the upper class, but he rejects intellectualism alone as the way of finding wisdom. He also abandons his family, choosing instead his shadow Govinda as partner in his religious journey.
In part 2 (chapters 5-8), Siddhartha could have been a wealthy merchant: he owned a house and possessions. Also, he has a lover, Kamala, who complements him even better than Govinda. Yet, he rejects the world of the senses by leaving her bed.
In part 3 (chapters 9-12), Siddhartha learns to listen to nature (the river) and thereby achieve nirvana. Siddhartha emulates the old river ferryman Vesuveda here. The old man is a mentor and spiritual guru, yet he holds no spiritual title and has no followers. The river is a symbol of renewal, peace, and the spiritual journey. It's flowing water is symbolic of the inner peace that flows within the spiritual man.
So, to review, here are the places, ideas, and people of import:
1. School - Intellectualism - Govinda
2. Home / bed - Senses (Pleasure) - Kamala
3. Nature / river - Nirvana - Vesuveda