It has nothing to do with balancing the game and everything to do with what Fairies ARE. I've seen so many complaints about Fairies resisting Bug, and what I must say in regards to this is that Fire resists Fairy for the same reason that Fairy resist Bug. Not because of damn game mechanics. It is due to the fact that Fairies are mythical beings and only Fire can resist their magic. Steel resostsa them because if you take a pixzie and rub her on a silver platter, she will die. And fairies don't have a strong enough immune system to deal with Poison. Fire, though, is an entity that eats things where what it eats cannot be restored. Fairies thrive where they can restore...and poison prevents this, fire scars and demolishes, while steel sears Fairy flesh itself.
Now Bugs are on the other end of the line. They are to embedded in this world for their moves to do much damage to Fairies. Fairy resists Fighting because attacks with fists and use of brawn in excess is too much embedded in this world as well. As for Dark...Fairy resists that because it represents dark forces that inhabit actions committed by humans.on't jus
There's explanation enough of you exercise your brain. Honestly, Pokémon makes much more sese than the fanbase seems to claim. I'm fairly certain GameFreak spent a plethora of meetings making sure the had the types right.
I know that the type chart has been made to balance the types and to make sense but not everything in it is completely logical. Why does the dragon type resist the water, grass and electric type; why does the steel type resist dragon but not dark; why does poison resist fighting but not normal. why does steel doesn't resist dark and ghost but it doesn't psychic; why does ghost resist bug and poison.
My point is that yes the fairy type was included because it has some logic behind it but remember that it was also created to balance types. Although the type chart is quite balanced, it could also need some minor changes.
I didn't really research anything at all so maybe some things do have reasons to back it up.
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