The higher the rarity of the character the higher the tier he will be able to obtain. Not only will the character be able to access higher isekais, but he will also gain an experience boost the higher the tier gets. The higher rarity a character has, the higher tier it will be able to obtain. Not only will the character be able to access more isekais, it will also gain an experience boost depending on the tier.
im.upgrade [local id]or
im.u [local id]
Once you upgrade a character it is irreversible.
6* characters which are obtained from boosting are not upgradable.