Someone who is always trying to demonstrate their talents.
show-off
A person, especially a child, who secretly tells someone in authority, especially a teacher, that someone else has done something bad, often in order to cause trouble
tattletale
Someone who is constantly making others feel bad or dampening the mood of a group with negative comments
Debbie Downer
Unlock this slideshow and over 4 million more with Baamboozle+