For your puppet character to be effective in the classroom, it's advisable to create a backstory, or a 'puppet legend' (the word legend is often used in spy novels – the the Spy Museum's language of espionage define it as ‘A spy's claimed background or biography, usually supported by documents and memorized details’).
One of the ways you can do this is to use a puppet passport. The one that I use is pictured here – it's from the 'The People’s Republic of Merseyside' (as I am!) and it looks like a real passport. Inside, all Charlie’s details have been filled in, there’s a photo of him, and even stamps from the places he's visited. This really helps to develop the puppet's backstory.
Here are some things to think about (and possible questions that your students might ask you about your puppet) in terms of creating a good, solid backstory.
Basic Details Age, birthday, job – though normally when working with infant or primary students, I say that Charlie is the same age as they are. Personality traits Is s/he shy, outgoing, happy, enthusiastic? Does your puppet have any catchphrases? One of my puppets (Dudley) loves chocolate and whenever he doesn't know what to say, he answers "Chocolate!" to every question. Family Details Does your puppet have a mum or dad/brothers and sisters? If s/he does, do you have any family photos? Living Details Where is your puppet from? (I usually choose an English-speaking country to encourage the class to speak English to it, of course.) Where does your puppet live now? At school? With you? Who does the puppet live with? What type of building does it live in? A flat? A house?
Hobbies / Likes and Dislikes Does your puppet get up early/late? What does your puppet like to eat? What does your puppet love? What are your puppet's hobbies? What does s/he watch on TV?
Here’s a link for play passports you can print off: