In the Kingdom Hearts series, which is not an optional boss you can fight?

Master Yen Sid is not an optional boss you can fight in the Kingdom Hearts series because he serves as a wise mentor and powerful sorcerer, playing a major role in guiding the heroes and assisting them in their quests. Although he possesses great magical abilities, he maintains a peaceful and neutral position, focusing on protecting realms rather than engaging in battles. Consequently, players cannot encounter him as an enemy to face in combat, emphasizing his important role as a mentor rather than a boss.