I had heard that Marineangemon was very popular among young children because, despite his cute appearance, he is a powerful Ultimate digimon. Thus, it was decided quite easily that he would be Kenta's partner.
The one problem was that the image that we received from the illustrations was so strong that the character's image seemed to be already held by all of the staff, as a sort of consensus. After seeing the new opening sequence(for the latter half of Digimon Tamers in Japan), I felt that the size difference between Marineangemon and Kenta was very large.
I spoke with Director Kaizawa, and we re-thought what sort of character MarineAngemon would be. It was decided that, in the Japanese version, Marineangemon would not speak like the normal characters.
Ms. Ai Iwamura was a very young woman who played Marineangemon as her debut role. Even a veteran voice actress would have had difficulty expressing emotions through whistles and squeaks, but Ms. Iwamura did her best, and I think that her own version of Marineangemon was perfected by the last episode.