Tigger is Mary Anne Spier's pet cat, which her overbearing father allowed her to get when he stopped dressing her himself. Tigger is a grey tiger-striped cat, hence the name Tigger (the tiger-like character from Winnie the Pooh).
Tigger is resentful towards Mary Anne as he feels she is overbearing and cuddles him too much. He wishes he had more freedom as his favourite thing to do is to climb to the top of letter boxes and see how far he can stretch his neck. Mary Anne is constantly telling people how cute he is. Mary Anne got him on her birthday in book #10 Logan Likes Mary Anne!. And she already knew how she wanted him to look. Logan helped Mary Anne pick out Tigger.
Tigger once left in Mary Anne and the Search for Tigger.