For favorite parts, I honestly have to say the Concept Art Gallery, and just generally discovering the developer history of this game. Idk why it was ordered correctly in the menu, but aside from that, I like the concept art because I -love-, and I mean, LOVE viewing old/scrapped material in a game, it satisfied both the side of me that loves preserving things so they aren't lost to time, AND the side of me that likes looking at developer history, something that on a side note is rich on the Hudokai Facebook page.
If I can go into details, the history behind the Sea of Seals arc is fascinating because there's so much pen to paper concept art that oozed 2000s energy, it made me feel like these were characters one of the leads had for a long time and wanted to make something out of it, but couldn't, so they added them into this story as a sort of tribute to their fond memories with them. It's cute, I really like it.
Additionally Max and Alex also intrigued me because early on we found pen to paper image of these two purple haired guys fighting, it looked cool and it reminded me of said 2000s energy of the Sea of Seals arc had, and it was constantly on the back of my mind as we went through the game. Then, when the game was done and it was time for me to look at the files, I found out that there was this cut character named Alex who looked exactly like one of the guys in that image, and the fact he was scrapped in favor of making Max a main character. The most fun part of that, and frankly everything revolving the history of this game is piecing together parts of the game that seemingly are innocuous, but when put together reveal something grand, ya know?
As well as finding out reasons why certain things look the way they do. Nova's armor being shading differently than his body is cause of the fact the design was traced over his old scrapped design, for example, pieced together by looking at the concept art and Facebook posts from 2012.
There are so many lil' easter eggs and stuff in this game and I love it~ Genuinely fun, no strings attached (p.s. that birthday cake should've been hidden in the files lol, it's so cute)