Below is a list of some places where you can obtain hero biometric to get more heroes or rank them up.
Daily Login
Players receive rewards depending on how many days they log into the game and every so often there are weekly login rewards that offers hero's biometric.
Elite Stages
You have three attempts a day to get biometrics and they become unlocked once you finish their normal counter part. Starting from the first stage they must be 'unlocked' one by one. The biometric you receive will be based on the boss you fight within the level.
Honor Shop (Timeline Battle)
When you fight inside the timeline battle or the arena system you will be rewarded honor tokens which can be used to exchange different heroes. The roster in the Honor shop is rotated every 3-6 months so save your honor points if the hero offered at the time is not to your liking.
Chaos Token Shop
Chaos tokens are earned in Villain Siege when you kill the boss. Villain Siege is one play where you put your entire hero roster against bosses 1 by 1. Chaos tokens can be used to exchange for hero biometrics in the Chaos Token shop. Like the Honor shop the roster is rotated every 3-6 months so save your Chaos Token points if the hero offered at the time is not to your liking.
Purchase via Crystal
The are several options in the shop section to get heroes including the 'flash' sale where if you have the required crystals you will be able to get hero biometrics at a good discount.
'Special' Chapter
The game offers you a special chapter where you can fight to win an 'event' hero.
Dimensional Rift
Every so often when you complete Normal or Elite stages you will get Dimensional Rift encounters which are stages that allow you to get additional hero biometrics. The higher levels offer better hero biometric drop rates.