Joker is a kind, easy-going, and cheerful  individual, who strives for the mirth of others, as shown when he intentionally drops balls he was juggling on his head for the amusement of the audience. 
Despite this outer appearance, though, he is very serious about his duties and does not easily give in to emotions.

Joker displays great care for the other members of the circus, but puts pleasing Baron Kelvin, no matter how abhorrent he finds his requests to be, above them.