I like to go for a multi character style, but I do have a "main" or character I am most comfortable with. He was mainly a side character for me from INJ1 but when I saw his new moves in his breakdown I knew I'd be comfortable with then with his old turbine and rocket shooting self.
But I know that a character single character rarely ticks all the boxes so I choose a character for very type of counterpick situation.
Before I went online I went through every character in practice mode, even the ones I didn't see myself playing in the future at all.
After bring 2-3 characters I liked from Week 1 practice to online I played more characters I knew with fill the gaps that I didn't get used to initially
To date my characters for dealing with matchups are:
Green Lantern: previous main makes him most comfortable pick, good well rounded character and not a bad choice to test the waters of an unfamiliar matchup
Supergirl: counter zoned with her mobility, heat vision and teleport punch
Green Arrow: similar to Lantern but with better wake ups, different zoning, and not as meter dependent
Atrocitus: go to rushdown character thanks to his good mids, pressure with Dex Star, and damage
Deadshot: go to zoner simple as that