Nick Clegg has praised Charles Kennedy for "having more political talent in his little finger than the rest of us put together".
The former Lib Dem leader said it was "desperately sad" that Kennedy died so young aged just 55.
He said: "Charles was the absolutely opposite of your cardboard cut out point-scoring politician.
"He was funny, but also very thoughtful. He was tremendously kind but very courageous too.
"I think he spent over 30 years in the House of Commons but throughout that he remained a totally decent human being. I think lots of people thought they knew Charles Kennedy."
Kennedy was forced to stand down as leader of the Lib Dems, paving the way for Clegg, after admitting he had been receiving treatment for alcoholism in 2007.
Asked about that period, Clegg said: "Charles Kennedy on form, on a good day when he was feeling strong and happy, had more political talent in his little finger than the rest of us put together.
"That's why everyone felt it was just so tragic to see someone with such huge gifts also struggle, as many people do, with the demons that clearly beset him and the problems that he acknowledged that he had with alcohol."
Clegg said he had been in touch with Kennedy just a few days ago and despite recently losing his Ross, Skye and Lochaber seat to the SNP, Kennedy still had politics at the forefront of his mind.
"He was already thinking about how we should make the case for Britain remaining in the Europe.
"He had as much appetite for those kind of debates as the first day I met him many years ago."