copycat
someone who has few ideas of their own and does or says exactly the same as someone else
chatterbox
a person, especially a child, who talks a lot
slimeball
a very unpleasant person whose friendly manner is not sincere
troublemaker
someone who intentionally causes problems for other people, especially people who are in a position of power or authority
daredevil
someone who takes unnecessary risks
fat cat
someone who has a lot of money, especially someone in charge of a company who has the power to increase their own pay
night owl
someone who prefers to be awake or active at night
cheapskate
a person who is unwilling to spend money
to be (as) good as gold
(of a child) to behave very well
heart of gold
someone with a kind and generous character
a whizz at/on/with something
someone who is very good and successful at something
cold fish
someone who seems unfriendly and who does not share their feelings
pain in the neck
someone or something that is annoying or difficult to deal with
class clown
someone who behaves in a silly way, often intentionally, in class.
black sheep
a person who has done something bad that brings embarrassment or shame to his or her family
backseat driver
a person who gives unwanted advice or criticism, esp. to the driver of a car
Couch potato
someone who is lazy and watches a lot of TV
fair-weather friend
someone who is a good friend when everything is fine
early bird
someone who likes getting up early in the morning
to be (as) nutty as a fruitcake
to be a very strange or crazy person
armchair critic/gardener/traveller
used to refer to a person who knows, or says they know, a lot about a subject without having direct experience of it
oddball
a person whose behaviour is unusual and strange
crybaby
someone who sheds tears frequently and easily.
be a barrel of laughs / fun
to be funny or enjoyable
worrywart
a person who often worries, especially about things that are not important
old as the hills
someone who is very old
busybody
a person who is too interested in things that do not involve them
Jack of all trades
someone who can do many different jobs or types of work