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When you first start up the game, it asks for an age between 00 and 99. Spoiler:This age choice does not refer to the player, but to the main protagonist Marina Liteyears, and has an effect on one frame in the true ending of the game. If Marina's age is set to 15 years or younger, her true human form will be physically closer to that of a child. If her age is set to 16 years or older, her true human form will be closer to that of an adult.
Contributed by MehDeletingLater
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At the end of the game, Spoiler:the game's three previous bosses merge their mechs together to form "The Beastector", the final boss of the game. The mech's design seems to pay homage to a few notable Japanese mechs, such as the Gundam robots from the Mobile Suit Gundam series. Similarities include the distinct V-shape on both of their heads, as well as the use of a beam rifle similar to the ones from the Gundam series.

The way each of the individual mechs morphs into a single bipedal mech is also reminiscent of the Megazord from Super Sentai (Power Rangers in the west).
Contributed by Castle23clash
In one of the stages, there is a mini-boss named Calina. Before fighting her, a cutscene plays where she takes a minute to transform. This is a parody of the magical girl anime Sailor Moon and its transformation scenes.
Contributed by Castle23clash
On the title screen, by holding L, A, Left C & Right C, and pressing Start, it's possible to access a Sound Test mode.
Contributed by SOGESNAKE
According to Treasure's CEO, Masato Maegawa, in an interview conducted by N64.com, there was at a maximum 15 staff members working on the game and an average of 12 throughout the development.
Contributed by GorgonBull35