13 Sentinels: Aegis Rim is split into three parts: Remembrance, Destruction, and Analysis. The first two are pretty self-explanatory: Remembrance is 13 Sentinels: Aegis Rim's story mode, while Destruction is the game's mech combat.
Analysis, by contrast, isn't so much played as it is researched. It's basically an in-game encyclopedia, which is useful for keeping track of 13 Sentinels: Aegis Rim's sprawling narrative.
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It's not all available at once, however. Each topic is a separate mystery file, and each one needs to be unlocked, either by fulfilling specific conditions or spending mystery points.
The mystery files serve multiple purposes. They partially function as a recap system. If a player forgets what a certain character is doing or what a certain plot element is, they can look it up in its corresponding mystery files. Each file gets updated as the player learns new information, and sometimes there are tidbits of extra info that isn't available in the main story.
As part of 13 Sentinels: Aegis Rim's localization, the mystery files also explain Japanese cultural details that might get lost in translation. For example, the brawler Nenji Ogata calls the schoolgirl Tomi Kisaragi «Dogface,» which she's slightly confused by. The mystery files elaborate that he's using «Chinkusha,» which is seriously outdated slang.
Lastly, the mystery files give Vanillaware an avenue for its habit of lovingly rendering video game food. If a character is seen eating something or even just mentions it, then there's a guarantee that players will find an entry for it accompanied by high-definition sprite work.
Not all mystery files need mystery points to unlock. The most critical
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