[Deepsea]

I think an important thing, at least for me personally, is that the form represents something to the character. I think some of the best examples of this are Liner form, and Xtreme.

Liner form represents ryutaro standing up on his own, but still having his friends close by. It shows his growth as a character while keeping with the theme of teamwork and coming together.

Xtreme represents Shotaro's and Philp's relation ship, now they arent just two people in one body but two bodies acting together in harmony.

Mugen is another example of this since it reflects Takeru's philosphy about humanity and gains its power from reinforcing those themes.