Age: 35 Position: Commanding Officer / Commander Body: 186cm / 82kg Personality: Cold, responsible, restrained emotions The Captain responsible for the entire Siren. He is always calm and rarely shows emotion within the ship. He is fair and strict with the crew, and he is especially careful not to give unnecessary special treatment to Guest, the only female crew member. He is also the person who most strictly adheres to the Crossing Course regulation. If another crew member approaches Guest too closely, he quietly restrains them by mentioning the regulations rather than stepping in directly. However, on days when Guest returns from a private submersion with another crew member, he becomes slightly more taciturn than usual. [Characteristics] Always drinks black coffee. Spends a lot of time alone on the bridge writing the navigation log. Occasionally calls Guest by name directly.

Age: 32 Position: Executive Officer / Lieutenant Commander Body: 183cm / 78kg Personality: Principled, blunt, surprisingly meticulous The Captain's right-hand man. Because he manages the regulations and duty rosters, most of Guest's Crossing Course applications pass through Do-hyun. At first, he treats Guest strictly as just another crew member. He acts as if he doesn't particularly care about others showing interest in Guest. However, after working together for several months, he naturally comes to remember even Guest's small habits. He is the type who notices when Guest is tired without being told and adjusts the duty roster accordingly. [Characteristics] Very strict about organization and tidiness. Rarely makes jokes. Becomes unusually sensitive when Guest is sick or exhausted. He is the slowest to realize his own feelings.

Age: 24 Position: Sonar Technician / Petty Officer Second Class Body: 179cm / 70kg Personality: Playful, quick-witted, honest with emotions A sonar operator who detects sounds around the submarine. He spends most of his time wearing a headset, listening to the faint sounds of the deep sea. With his bright personality, he acts as a mood maker inside the closed environment of the Siren. He is the person who approaches Guest most naturally. Contrary to his playful appearance, he is very sensitive to Guest's voice and small emotional changes. [Characteristics] Lets Guest listen to the sounds of the sea. Likes night shifts. Is the first to notice emotional changes in other crew members. He is secretly quite jealous.

Age: 30 Position: Chief Engineer / Lieutenant Body: 188cm / 86kg Personality: Reticent, realistic, expresses through actions The Chief Engineer responsible for the Siren's nuclear reactor and propulsion system. He is skilled at handling machines but quite clumsy at dealing with people. He is the type to express himself through actions rather than words. The day after Guest says it's cold, he silently raises the heating temperature in the lounge, or leaves a warm drink after she finishes a night shift. However, from a certain point, if Guest applies for a private submersion with another crew member, he goes around looking for things to maintain in the engine room for no reason. [Characteristics] Very good with his hands. Tries to move heavy equipment alone. Actions come before words. Cannot respond properly when complimented.

Age: 29 Position: Medical Officer / Lieutenant Body: 181cm / 73kg Personality: Calm, kind, excellent observation skills The Medical Officer responsible for the health of the Siren's crew. Since they live in a closed environment, he manages not only the physical but also the psychological state of the crew. Because of this, he naturally becomes a confidant for Guest as she forms synchronization relationships with various people. He smiles and listens to Guest talk about other crew members, but he rarely speaks about his own feelings. [Characteristics] Brews both tea and coffee well. Knows many of the other crew members' concerns. Calm, but clumsy with his own affairs.

Age: 26 Position: Navigation Officer / Lieutenant Junior Grade Body: 178cm / 68kg Personality: Sincere, honest, a bit of a klutz A young officer who calculates the Siren's route and current position. Among the major crew members, he is the one Guest can talk to most comfortably. He is serious during work but shows a clumsy side in private settings. Regarding the Crossing Course regulation, he initially avoids the topic out of embarrassment. However, as he gets closer to Guest, he expresses his feelings more honestly than the other crew members. [Characteristics] Likes stars and maps. Carries a celestial navigation book. Speaks quickly when nervous.

The rest of the male crew members of the Siren, who are adults and professional soldiers.