Senta Umino serves as a teenage crew member aboard the reconstructed submarine 707, joining alongside fellow trainees Kenji Minahaya and Goro Kusaka. He possesses an overweight build and a distinctive snaggletooth. As a junior sailor, he undertakes standard naval duties under Captain Hayami's command during missions against the USR terrorist organization. This assignment follows his witnessing of the original 707 submarine's destruction during an attack on the Peace-Keeping Navy's inaugural ceremony. His operational contributions remain within the established crew structure.