Who are essential characters

You get more out of the game if you concentrate on rebels and scoundrels.
You'll get credits and training droids.
That's necessary to level up the toons you want to have.
Don't lose time and go all in on:
Ackbar, StHan, Lando, Leia, Biggs, Wedge and who else is free2play.

You'll also get a 7* Palpatine, whenever the event comes back.
After that you need 5 Jedis for Yoda
So QGJ, Anakin, Luminara and so on are necessary.
To get Gear you have to have 7* toons.
Therefor the Ewoks are strongly recommended (Teebo(!), ewok Scout, ewok elder)
The next raid is easier with Jawas/Jedis
And now, when you get everything you need the best mods.
Any 7* toon helps.
Or Jedis, Jawas, Resistance, Rebels, Scoundrels,..
So yea, that's basically the road to follow for max. effort.