Mercurial means he's lively, witty and not a boring stick in the mud. 'He's a character' means he's got a unique personality. He's fun to be with.
POTUS not dissing KIM at all, here. He's saying he enjoys him.
adj. Having the characteristics of eloquence, shrewdness, swiftness, and thievishness attributed to the god Mercury.
For those who never had Mythology in school.