Risto Ryti - Is he seen as a Hero or a Villain in Finland?

He was voted as the second greatest Finn after Marshall Mannerheim some years ago. He sacrificed himself and his reputation to save his country. I think that tells you enough. Though it didn't happen exactly as you wrote. It was a scheme agreed with Mannerheim and maybe others. They all knew the constitution.

The "war quilt" trial was a farce and everyone knew it. The Allies demanded that they were sentenced even if it was against all principles of justice and made mockery of them in a democratic country. They were not even allowed to defend themselves because then they would have mentioned the Winter War and who started it. They went along with it because the other choice was a trial by Soviets.

When Ryti and others arrived to the Christmas service in prison's church, the officers serving their sentences for the Weapons Cache Case stood up in respect.

Finland sided with Germany because siding with Russia would ultimately turn into a forced Soviet state, like many of the European countries who sided with Russia ended up as. Germany's alliance with Finland ultimately is what ended up with Russians attacking the Germans because of the weapons Germany supplied Finland.

He's seen as a hero in Finland.

I was taught in school that he was only put into jail because USSR demanded so.

The general opinion is that Ryti did what he had to. There wasn't much choice.

The answer lies in the political issues of the time. Nazi Sympathiser or a socialist.