The main challenge of Quest Mode is to beat it in the fastest time possible during the first run for the leaderboard timing. Grinding can make characters overpowered, but it obviously takes longer. However, there is also another end game in this mode and it is the final perk called Parallelism. This perk allows the overpowered Quest Mode character to be used in Arcade Mode and helps anyone who would like to tackle those harder badges!
Of course, using custom characters in Arcade Mode disqualifies the score from the leaderboards. And for those who want to crank up the Arcade difficulty, nothing is stopping builds that are significantly underpowered. The perk can be activated as many times as there are fragments to spend, although, only one custom build can be registered for each of the three characters. A new build will replace the previous one. So go set up those custom characters and play Quest Mode more!