Ru is the central philisophical idea of the Shintaru specifically and the Vongta in general. It is described by Lord Blackstone this way.
"It's one of those words that doesn't translate well to Glorin. It's used like destiny or fate but it's more than that."
Captain Blackstone scratched his head and searched for the right words
"So...imagine that the universe is a large river and we're all travelling on it. You can go to the bank but everyone will leave you behind. You can try to go against the current but you'll create chaos for yourself and those around you. Ru is going with the tide. It's right action. It's about knowing your place in the universe and doing what you're supposed to do."