It’s an adjective and it means not having or lacking enthusiasm. If someone is unenthusiastic about something it means they are feeling indifferent, apathetic, passive, phlegmatic, lukewarm, listless, languid, lethargic, emotionless, perfunctory about it.
Undertale is a game created by indie developer, Toby Fox. In it, you can choose to either spare every entity that appears before you, or slaughter them mercilessly. It has multiple endings, a fantastic soundtrack, and although it doesn't have the best graphics, its story, wittiness, and humor make it a very enjoyable game.