I was thinking one of two things, but first, you're making a baked cheesecake layer or a no-bake custard-y type of cheesecake?
You could do exactly as NorCalBaker suggests and bake the cheesecake, with crust, in the Hello Kitty tin, and just let it flatten out as well as possible. Or you could get it out of the tin and then, well chilled or frozen, level it with a serrated knife, which would be butting the raised face (and crust) off.
I was originally picturing setting the Kitty cake on top of a larger, square or oblong or bigger round cheesecake. The Kitty pan, which if it's the Wilton one is 10.5 by 9.25, so a bit too big to sit atop a 13 x 9. However it's 2" deep, so here was my other thought.
If you're making a no-bake cheesecake, thickened with gelatin, could you bake the cake as planned, remove from the tin, perhaps trim it a little to take the exposed top layer off, then, with the pan cleaned, carefully line it with plastic wrap, replace the cooled firm (chilled) cake, and pour the no bake cheesecake to the top of the pan, even extend the pan depth with a little foil rim if necessary, chill, and invert?
You'll want to definitely well grease the inside of the tin so you can remove the baked cake. I've only recently learned about making a homemade "baker's secret" consisting of equal parts vegetable (or other flavorless) oil, solid shortening, and all-purpose flour. Brush that on evenly and not sparingly with a pastry brush and that should help. As NorCalBaker detailed in a thread about what's harder, baking or decorating, you'll definitely want to make sure the cake component and the cheesecake component are very well chilled if you have to do finagling with them.
I Googled, here is a pic of the Wilton Hello Kitty cake on top of a larger cake, which in your case could be the cheesecake.
http://www.wilton.com/hello-kitty-birthday-cake/WLPROJ-6228.html But note this is atop a 16" round 2-layer cake, which is bigger than most of us have pans for.