Upgrading characters is a way to make it gain experience faster and allow it to use higher level isekais. The higher the characters rarity, the higher the tier it can gain. A 1-star character can get maximum tier 1 while a 6-star can get maximum tier 6. See below for the cost of upgrading and the boosts it gives.
im.upgrade [local id]
or im.u [local id]
im.u 231
Once you upgrade a character it is irreversible.
7-star or higher characters which are obtained from boosting/donating/giveaways can be upgraded to tier 5.
Upgrading a character will revert it to level 1.