Prosecutors in Madrid have filed a complaint to a court in Pozuelo de Alarcon in which they accuse the Real Madrid star of a "voluntary" and "conscious" breach of his tax obligations, totalling €14.7million.
The situation has led to claims Ronaldo wants to leave the European champions, potentially prompting a scramble for his signature from the world's biggest clubs.
Santos, who led Portugal to Euro 2016 glory in France, insisted the 32-year-old's character should not be questioned and launched a passionate defence of his star striker.
"I've known him for many years," he told a news conference ahead of Portugal's Confederations Cup opener against Mexico in Kazan.
"I was his last coach in Portugal when he was 18 years old. I can put my hands on the fire for his character and integrity, as a man and an athlete. That's my clear statement. I trust him 100 per cent as an athlete and as a man."
Santos was pressed on the subject further but preferred to focus on on-field matters, adding: "I'm sure you want to ask the same questions but I will answer no more questions about Cristiano Ronaldo.
"I have a pact with my players, I have a great relationship with each of them. We've laid down that everything to do with their personal relationships is done outside the team.
"When we're focused on a tournament, all those other issues, we don't pay attention to.
"So you have to pay attention to what the sources are for this information. I never heard about some of the news being spread. Either I am deaf, and I'm not, everyone knows I'm a great listener, or you have to be careful about where this news comes from.
"Cristiano supports every single player in the team and we have a great relationship. There are no individuals. Each player supports the other."