It's not exactly heroic, but one option available to you in Pokemon Mystery Dungeon Rescue Team DX is to pinch things from the Kecleon Brothers as you explore dungeons. We'll explain how to steal from Kecleon and how - if at all - you can hope to beat him.
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How to Steal From the Kecleon Shop
Kecleon's mini-shops can be found randomly in dungeons past a certain point in the game, usually around the Frosted Forest and onwards. You can see their location on the map, or at last a good hint of their location: lots of items clustered together with a pale blue NPC icon next to them (that's Kecleon). There's a throwaway joke at one point that Kecleon is just hoarding items found in the dungeon and flogging them to the next person who comes along, so I guess it's not proper stealing…?
The process is pretty simple if you're just buying and selling: drop any items you want to sell on the carpet, and walk onto any items you want to take, stuffing them into your bag. Kecleon will ask for the money when you try to leave, and if you say no, that's when he panics and starts yelling about thieves. Things are about to get very bad.
See, Kecleon is arguably the most powerful enemy in the whole game, which is odd, considering he's just a shopkeeper. He's got stats that'll bring down pretty much any character in one hit, can take multiple actions for every one you make, changes type according to the last move that hit him so he's never got a lasting weakness, and - just to make things worse - the dungeon floor immediately begins to fill with his brothers after robbing him, all of which are equally powerful.
If you can make it to the stairs and progress down, you'll get to keep the items you took and the flood of uber-Kecleons will be over. However, that's very unlikely, considering how many of them will be prowling the halls and able to beat you in one shot.
How to Beat Kecleon
You cannot beat Kecleon. Alright, maybe somebody can, with a team of level 100 Pokemon and a perfect strategy, but if you're not that, you won't win. Bring down one of them, and there'll be six more already on you. They're like bloody Xenomorphs in that respect. They even know the move Bind, which'll hold you in place for a while until others show up.
No, if you want to pull off a successful heist (the odds of which are very small) you'll need to evade and navigate Kecleon and his brothers. Items like Warp Seeds are a good opening move: if you're near the stairs when the heist begins, throw one at Kecleon to try and send him across the map. If the stairs are far away, eat it yourself in the hope it'll put you close. Warping away Kecleons is a good strategy, and failing that, a move like Discharge which has a chance to paralyze everybody in the room can be helpful. You can't win a fight with them, so stop the fights from happening until you make it to the stairs. Then, if you've somehow survived, you can sit proud atop your ill-gotten loot… which probably wasn't worth that much in the first place.