Who might I have to defend my title against?

Solution 1:

Serebii has compiled a challenger list. They state that there are 10 potential challengers you will face, chosen at random. This is excluding the first time you beat the league which is fixed. According to Bulbapedia, the first title defense (second time in the league) is also a fixed challenger. And one challenger only appears on a certain day of the month.

Challenger names listed in spoiler below.

Hau (First title defence/second time at the league)
Sophocles
Ryuki
Gladion
Molayne
Plumeria
Hapa
Faba (Always appears on the first of the month)
Tristan
Kukui (First time in the league)