This is Sword Art Online. In this anime, game, and manga Kazuto Kirigaya loves video games. Upon being a beta tester in the game SAO, he enters it along with other thousands of players and soon discovers that he and the other players are trapped in this game until someone completes the game. Not only that, but if you die in the game, you die in real life. Thus Kirito(Kazuto's game name) swears to stay alive until the game is completed. While doing so, he makes a lot of friends, creates his own harem, and even falls in love with the beautiful Asuna Yuuki.