Do you already have your favorite characters to give the costume to? do you have any heroes whose costume you won’t give? Do you have any plans on how to use the costumes? Personally, I have 2x brienne. I think it’s great to combine them at a 3 * event. One of them will be in a costume, the other without. It will give a great combination of attack and lowering defense. As for the minuses, I will definitely not give a BT 4 * costume, as for me, the costume will make it worse. I might be wrong? I am very happy to read your ideas, advice and suggestions about who is worth it and who does not wear a costume 
I apologize in advance for my English (translator)
I think, if nothing else, most if the costumes are worth using for the stat boost.
So for BT, even if you don’t use the costume, level it up for the stat boost. The monk healer will be handy too for class trials 
Same thing with Li Xiu. Even if you don’t use the costume, level it for the stat boost.
Skittles is probably the only one I’d say isn’t worth levelling purely cause the original isn’t great to start with 
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thank you for the quick reply could you explain exactly what statistics are there? I’ve already got all the heroes listed to the max. Will giving them maximally made costumes make any bigger differences?
Boost is the costume bonus.
Check the FAQs for more, specifically Question
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Even if you dont use the costume they are still worth maxing imo for the boosts that they give the regular hero. 5%att 5%def 10%hp and 5% mana. Plus costumes max quicker and cheaper then regular heroes
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