The central twist of "The Invisible Guest" is a masterful piece of misdirection. Throughout the film, the audience believes that Ana Wagener's character is Virginia Goodman, a highly sought-after defense attorney hired to help Adrián Doria. The final revelation is that she is actually Elvira Garrido, the mother of Daniel, the young man Adrián killed in the hit-and-run. The entire consultation has been an elaborate ruse orchestrated by Elvira and her husband, Tomás, to trick Adrián into confessing to both Daniel's and Laura's murders.
The entire film is re-contextualized by this reveal. Every piece of advice "Virginia" gives Adrián is a calculated psychological prod to get him to reveal more incriminating details. When she suggests framing Tomás for Laura's murder, she is testing how far Adrián will go. When she has him mark the location of Daniel's submerged car on a map, she is obtaining the evidence the police could never find. The pen she uses is a hidden microphone, recording his entire confession.
Adrián's final confession is that after the initial accident, he discovered Daniel was still alive in the trunk of the car just before he pushed it into the lake. He drowned him. He also admits to killing Laura in the hotel room because she was becoming consumed with guilt and he feared she would confess everything. The film ends with Elvira removing her disguise in the apartment across the street as the real Virginia Goodman arrives at Adrián's door, just moments before the police, called by Tomás, come to arrest him with an irrefutable, recorded confession.