Is there any way to increase the likelihood of ES drops?

I played this game back when it first came out and put in like 16 hours of gameplay (according to Steam), where I cleared the story mode once. I started with Scarlet Witch and through drops unlocked the other 3 free characters. I also randomly got a drop for X-23.

I reinstalled the game after hearing about how great the 2016 version is, but unfortunately I did not get a free character, which makes sense because I already have 4 technically.

Problem is, I dislike all 4 characters I have. I don't think they are fun to play (besides Scarlet Witch), they aren't cool, and they aren't Marvel characters that I love by any means.

I played for 2 hours tonight, just burning through the story with X-23 (I really do not like her playstyle lol) and the entire time, I only managed to get 3 eternity splinters.

Is there any way to increase the likelihood of getting them? Are there any things I can do in-game? I would pay for a hero but as of now, if the "free" elements are this elusive, I don't know if I'll bother making the investment.