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How would you characterize the 18 types in Pokémon?
Psychic - They are the thinkers, after all. Psychic pokemon are often fragile, but can be absolute powerhouses when utilized at the right moment. This type also has a lot of tricks up its sleeve, like sleep, confusion, and healing, making it useful in lots of situations.
Dark - These pokemon are often tricky and always seem to be one step ahead. While psychic pokemon are the thinkers, I think dark best utilizes the 'mastermind' name. Something like Zoroark's illusion ability requires a lot of careful set-up and planning.
Flying - Flying pokemon are very versatile, as they have almost every possible Flying/Other Type combination used and are very often dual types, allowing for a ton of creative possibilities. Also, most of them are able to learn Fly, which allows you to go instantly from one town to another. No matter how many ideas you have for things to do in all these different towns, you can get there instantly with a flying type.
Steel - Steel types are often great team leads, due to their normally high defenses which make them difficult to take down in one hit, as well as the set-up moves a lot of them have (Stealth Rock, Spikes, etc). Steel isn't quite as versatile as flying, but much like flying you have a variety of dual types to pick from. Because Steels are such great leads, you have to plan carefully and judge which dual type is your best choice so you can get off to a good start.