I've been playing this game for quite some time now and did a pretty good reroll experience.
- 1st Gacha Valiant
You'll get a valiant on your first role: Darrion(defender), Shizu(Shadow), Keira (Mystic/Mage), Lucille(Healer), Freya (Warrior). You'll get a random faith as well which diverses the stats. - Farm all the way and complete trials to get your first 400gems.
Completing the 400 gems is crucial as it will lead you to a 4star hero. This 4 star hero will allow you to progress 3rd map. - Rerolling account. Because of the RNG, rerolling your account will give you a different gaming experience as the it requires different play style. Try and you'll enjoy!
