Each characters class has more than 5 avatars and each avatar has different moves, relic, passive skills, and stats. To unlock an avatar you must have its essence, this can be obtained by getting 3-stars in Adventure mode stages, Labyrinth mode, Pinnacle of Akard mode, events, and as a PVP ranking reward. Once you have the required amount of essence you simply have to go to the 'Avatar' menu, scroll down the list and tap the avatar to unlock it. Avatars can be leveled up using potions. There are three different grades of potions; S, M, and L and they can be obtained from Adventure stages, Tower of Babel, Treasure Hunt, Guild boutique, and Honor store.
You can increase the star level and boost the stats of an avatar by escalating it, to be able to escalate an avatar you will require more avatar essence. To find out how much more essence you need go to the 'Avatar' menu, choose the avatar and then select 'Escalate'. If you look at the bottom-right corner of the screen that appears you will see the number of essences you need. Tap the 'Obtain' button if you want to knwo where you can get them from. Each time you escalate an avatar you will unlock a new passive skill. In the same menu you checked to see the number of essences you needed you will also be able to take a look at your passive skills by pressing the 'Skills' icon.
Evolving the avatar is also done under the 'Avatar' menu. When you meet the indicated conditions the corresponding passive skill will become unlocked.
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