If you do not want to spend real money purchasing Power Cores to can unlock the various characters you can always collect character shards to unlock and tier up characters. Character shards like many games in the genre are rewards you recieve for completing tasks and when you have acquired enough of them you will be able to add a new hero or villain to your roster instantly. You can get character shards the following ways.
Daily Login
You will recieve character shards every few days by logging in daily and completing the Daily Objectives.
Auto Win
When you beat the third stage of each Campaign chapter with three stars you will be able to use Auto Win on Campaign battles and there is also a possibility that you will get some character shards as a reward.
Events
You can unlock specific characters by playing a series of battles in an Event. Revisit active Events whenever you hit a level with a multiple of 10 to face a previously locked battle and the possibility of earning more character shards.
Arena Battles
Although you will not get character shards for Arena wins you will recieve Arena Credits at your Inbox every 24 hours based on your current rank which you can then spend on character shards of your choice at the Arena store. The selection of character shards for sale at the Arena store changes daily.
Blitz Mode
When you reach certain levels in Blitz milestones you will be rewarded with character shards. This typically starts from the sixth milestone. The Blitz store is is similar to the Arena store except it has its own currency and daily character shard selection.
Note: If you tap on any character’s shards you want to purchase, it will tell you how many shards you still need before you unlock or tier up that character.