CONTAINS SPOILERS

Early on you may find out that you’ve maxed your Sujimon Trainer Rank to 10 and wonder why it can’t increase further. You need to first complete the substory “The CEO of Sujimon” which is located at the corner of Aloha Bridge at Jack’s Hotel (see image). After doing so, you’ll gain the ability to rank up your trainer level and the max level for your Sujimon will increase.