While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
After maxing the characters to level 90 and unlocking all their cores, the next thing to do is get capsules, which come in three forms: red for attack up, blue for defense up, and green for health points.
These turn up as rare drops for story mode, difficult level events but also time trials! Epic Quest also drops quite a few, you may notice that some events drop certain colours more often, again can’t confirm until more data from other users is collected.
You can buy them in the Exchange Shops (Guild, Event, Infinity etc) and they can be bought with rubies from the secret shop or with real money from the General Shop, including the Topaz section. Note that some of the item tickets and loot boxes have a chance to drop capsules.
Affinity & Codex
After you fully buff your character to +297 (99 points x3) you can go further by upgrading the affinity level to 9 or 10, and then a further buff is applied through codex, which is given depending on how many characters you have and how many levels have been reached.