I already have 1 water mage and 1 earth mage i want another char (1 or 2)

so should i have a fire or a wind ?

If a fire Wit or Killer? (never used fire)

If a Wind Wit Or Killer? (never used wind)

i usually go for the healers/supporters (Water, Earth)

I was personally in the same situation, with 1 water mage, 1 earth hybrid (first char... old school) and a wind mage. Eventually if you make a wind, do it ATK/Killer type, it's crazy how fast they are and they can do such a big damage... If you want it to be sealer, then add on MATK. I have 3 fires more now, 2 Attack builds and one mage... I love the mage more :)
As an answer to your question, I suggest to build a fire, then later a wind :)

MATK affects the accuracy of seal skill ?
Nobody comfirm this with me.