Eventually, in order to secure the victory, the professors eventually decided to send Serena as Dark Moon's representative for the 1-on-1 battle.
Kain, while disgruntled by this decision, couldn't fault their reasoning. Unlike Serena, Kain had participated in more battles throughout the competition and revealed more about himself.
On top of that, while the buzz surrounding had faded for several months, they couldn't underestimate the intelligence network of the top ranked college, so they probably had even more information about Kain than he'd revealed at the tournament.
Serena, on the other hand, hadn't revealed as much within this competition. Not to mention, the variety of skills her Starweaver could display was a lot greater than Kain and the others, and many of them had never been revealed publicly, neither before nor during the competition.
Therefore, Serena presented a greater element of surprise.