GameMaker is your best bet. Its not a 'platform maker', if thats what you want, because there arent any 'customizable characters' or anything like that. You (or someone in your hire) will have to make all of the graphics. But platformers are easy to make with it, so you shouldnt have any problems.
There are no actual game makers (easy click, drop, enter values, and load images, animations) - game creation software designed specifically for 2d sidescrollers.
Although, there are 2d sidescroller engines, along with other 2d game engines that you can learn the scripting language in order to program and code your own story and game items, sprites, npcs, enemies, ect. Although I highly doubt there will be any 2d game engine with a scripting language powerful enough to let make any major changes to AI in enemies.
2d platform game maker
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