Lacke is a character in Let The Right One In. In the movie he is the fist one to suspect that Eli is a vampire. He

Lacke with Virginia

meets Hakan whom he knew has a young girl living with him.

It all started when his best friend Jocke whom Eli killed. He hears from his friend Gösta that a kid killed Jocke, so they went to the tunnel to see it for themselves. There he sees fresh blood underneath the snow. And there he promised to kill the young girl who was responsible for this.

His anger fueled once more when Eli tries to kill another one of his friends. He saves Virginia whom he has a relationship with. But Virginia is now a vampire and lives off drinking her own blood. She committed suicide much to Lacke's grief.

He sets out to find her and there he sees Hakan's apartment windows covered with papers. He went to Eli's room and finds her in a bathtub. He tries to kill her but he was interrupted by Oskar. And that's where Eli killed him after being awaken, much to Oskar's horror.