The monster, subbed as either Steve or Tony, is the main antagonist of HetaOni. In the game, he is the one that chasing the nations, actively attempting to kill them for some unknown reason in the mansion. He may be refered to as "the thing" in HetaOni. According to some of the nations, Steve remsembles America's friend, Tony, therefore explaining that name. It is however later confirmed that Steve is a different entity from Tony. Steve is just a random name. Its appearance is that of a giant, grey, naked alien-like enitity wearing a seemingly eternal serious expression.
Steve is deisgned like Ao Oni, particularly in their corridor apperance and their tendency to pop up randomly. Creepy music will start playing if Steve is following the user in the corridors or going to appear in the next corridors. Sometimes the figure of Steve can be seen in the black area of the screen outside of the corridors of the mansion, meaning that Steve is likely to appear when the user moves into another corridor. If the user runs into Steve, then it will be game over. Although the user cannot get Steve to stop following him, he can run in the opposite direction to lose him or hide in the 1st floor bathroom. The game is designed so that it is easy to outrun Steve, yet the user will need to be aware of his appereance so that he can start running as soon as possible.
The only times when the user can encounter Steve without losing is in the boss battles . The nations will use various fighting techniques and weapons in an attempt to defeat Steve. Users can choose to either Attack, Skill or Guard. The Attack mode can be used indefinitely without deducting any MP points , yet usually it is impossible to defeat Steve without using any Skills.
The Skills varies between different nations and costs different amounts of M points. Choosing the Skills mod will deduct the MP points of the nation by the number indicated next to the Skills. This can be revived by offering the character Beer, which can be selected in the Item section. When a character's HP point drops to 0, the character will be shown to collapse down with a skull sign above him. When all the characters battling Steve have 0 HP points, it will be game over. The H points can be increased by offering the natioins Onigiri during battle.
The user could select which members to battle against Steve by using the Formation function. The ones in the Battle Party are the ones fighting.
The user could earn HETA , onigiri and beer if the party was successful in defeating Steve.
Later on in the game, it is realized there are many other different aliens like Steve. Some are octopus-like versions of Steve and some are big blobs with many black, beady eyes.
After Italy escaped the first time they came there, he could not bear the thought of all his friends dead, and he made a pact with the monster and the book; should he manage to catch Italy and kill them all, he wins. He did actually succeed once, but England rewound time for him.