Diddy is a character that doesn't like to be perfect shielded. Banana's are stopped by perfect shielding then bounce conveniently within grab range. Just shielding a banana will result in a trip. His aerials all have deceiving range and power. He's difficult to edge guard directly since his Side-B can result in you being spiked. If you can get him to the point where he needs to Up-B though he's not nearly as difficult to keep off the stage. He's fast and he can follow up his dash attack with quite a lot of moves. He has trouble KOing on the ground though so use that to your advantage. KOes horizontally easier then vertically, so wide stages are good counterpicks against him.
Some characters, like Wario, can eat the bananas fearlessly and Sonic can spin dash over them to no effect. His bananas are his strongest point, removing them helps considerably. If you happen to grab one and can move fast, I suggest using them against him. If you move slow though your best bet is to just throw it off stage.
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