The two best characters to invest in are Asura and Artic Abyss Poseidon.
Asura is just extremely strong and very easy to play. Her skills allow her to dominate her foes by stopping time for them, which can give breathing room when things are dire.
Additionally, she is extremely fast and can quickly cancel attacks with dodges, making it easy to reposition.
Her range is nothing to sneeze at, either! Despite being a brawler, her attacks are disjointed and appear some distance away from her. On top of that, she covers wide areas and has great crowd control.
On the other hand, Artic Abyss Poseidon is a purely ranged character. She can summon icicles with her skills, which deal great damage and have a good Area of Effect.
Her ice skills will also freeze over enemies, dealing some extra damage and stopping them for a few seconds. She’s also pretty quick and has no trouble repositioning herself out of harm’s way.