Aka, the man who could wreck the eurozone's rescue plan?
That's the one; he is threatening to withdraw Finland's support for the second bail-out of Greece agreed at the end of July, which the markets are desperate for the eurozone's members to ratify.
What's his problem?
As part of the deal, Greece promised to deposit money with Finland as collateral for its share of the bail-out. More influential members of the eurozone complain that Finland is getting special treatment and they're threatening to veto the bilateral agreement.
What does Mr Katainen have to say about that?
He points out that his MPs backed Finland's involvement in the eurozone rescue plan on the basis of the Greek arrangement. Without it, he may have no mandate to continue supporting the deal.
Is he going to get his way?
Probably not. If Greece has to put up collateral for its latest bail-out, it's going to need to borrow even more money – if everyone else demands what Finland has been offered, the package just won't be sustainable.
So there'll be a compromise?
There will have to be. The good news is that Mr Katainen is a consensus-minded sort of guy. He has no choice: the Government he has headed since June is a six-party coalition.
He looks awfully young?
Yes, he's not 40 until October.
Does he know what he's doing?
People rate him very highly. He was a teacher before moving into politics and took over his party's leadership when he was just 33. Before becoming PM, he spent four years as Finland's finance minister – the Financial Times rated him Europe's best – so he knows a bit about the eurozone crisis.
And when he's not holding the eurozone to ransom?
Mr Katainen, who grew up in Siilinjarvi, in eastern Finland, lists his hobbies as hunting and preparing food from wild game. He's not as meek as he looks.