Houkago Adventure Valiant (1998)

Houkago Adventure Valiant

Daiki Okuma, while he's trying to confess his love to his beautiful schoolmate Mami Fukuzawa, sees a dimensional passage opening in front of his eyes, from which arrive a girl and a monster that, while fighting, hurt Mari.
The mysterious girl, Selphy, tells Daiki that the only way to save Mari is to take her on the other side of the portal.