What I'll post below is not exactly a full guide for the game, more like a collection of hints how to get each of the 7 endings (Silentman confirmed on reddit that there are 3 hidden endings) while trying to minimize spoilers:
When you choose what does the mech look like, it's much more than just an aestheic choice. The two variants are actually so different from each other that they reqire completely opposite strategies during the training missions, and because of this they have separate (though parallel) routes [Super Route and Real Route], each with its own good and bad endings (which together give us the 4 main endings). One JEET variant is a durable melee fighter that could (and maybe even should) take a more direct approach and even can afford being a bit careless at times; meanwhile, the other one is a fragile speedster with a ranged weapon and requires more careful fight strategies. Getting a good ending for your JEET variant of choice depends on whether you manage to choose training mission strategies more suitable for that variant.
The hidden endings are independent of the mech variants, though.
Also, it's one thing if you keep choosing wrong strategies (it will just get you a bad ending for your current variant of JEET). However, it is a completely different matter if you keep being downright rude, ignoring JEET and making them to think of mission strategies on their own - prepare for darkest'n'edgiest playthrough, resulting in a hidden (bad) ending [Coward Ending].
As for the other hidden endings... Did you check your message box at the start of each day? Someone is sending short mysterious messages, that actually are cryptic instructions for the training missions, except for the last message which involves some file... Following that last instruction makes that someone to reveal themselves, overriding any main ending you were going to get otherwise with one of two hidden endings - which one you'll get depends on whether you managed to follow all of the other instructions [True Ending] or not [False Ending]. Note: said instructions are cryptic enough that you'll need to carefully read the outcome of each possible training choice to decide which ones fit the instructions. Good thing is, these instructions are always the same regardless of the current JEET variant, and the bits of strategy outcomes they refer to appear for both variants as well (though might be presented a bit differently).
EDIT: Added names of routes and endings.