In Pokémon the Series: Sun & Moon, no character defeats Ash in the Alola League semifinals because Ash reaches the final without a semifinal loss.
The inaugural Manalo Conference uses a tournament structure in which Ash defeats Faba in the first round, Hau in the quarterfinals, and Guzma in the semifinals. He then defeats Gladion in the final to become the first Alola League champion.
This detail is easy to confuse because Gladion is Ash’s main final opponent, while Guzma is the rival he faces immediately before the championship match. Kiawe is also a prominent Alola battler, but he does not defeat Ash in the Manalo Conference semifinal.
Ash’s victory is historically significant within the series: it is the first time he wins a regional Pokémon League tournament. The Alola League’s inaugural status also means that the competition differs from earlier conferences, which were generally presented through more established regional League formats.