Edriel, Do Not Tell's main character, is a shy and anti-social student. The other students think he is weird, and the school psychologist is worried about him. She asks his favorite teacher, Mr. Ervin, to mentor him outside of class, but instead they end up resuming a romantic relationship they had earlier put aside. Things start looking up for Edriel even more when he becomes friends with a popular new kid, Alfred, who seems to share his interest in the history club.

Eliam Ervin, a youngish and handsome history teacher at Edriel's school, knows it's unethical to get involved with students. But his and Edriel's feelings for each other are so strong, they just can't help themselves. He is not ashamed to be with Edriel, but they still have to be careful not to be caught. Eliam is also annoyed by the obsession of his ex-boyfriend Andy.

Sid enjoys being with guys; at first he has a secret relationship with Edriel. But he doesn't seem to realize the importance of treating his partners with any dignity. He ignores Edriel in public because he doesn't want his friends to see him getting along with the 'weird' kid. Edriel finally ditches him, so Sid plots to seduce his straight friends Thiago and Fabien against their will.

Andy's grandma is the boss of 'the organization,' a secretive group with connections to organized crime. When she is away, he gets to be in charge, giving him an opportunity to use the organization's agents for his efforts to win back his ex-boyfriend, Eliam. He thinks that jealously abducting everyone that takes an interest in Eliam will somehow help with this... fortunately, he hasn't found out about Edriel yet.

Edriel's Mom, unknown to her son, works for Andy's organization. He does suspect something odd about her job because she always comes home late and is often tired.

Blanche, a practical and efficient woman who often wears red business dresses, is the assistant director of 'the organization,' . Disapproving of Andy squandering resources on his personal agenda, she is working to convince another person to take control.

Sandro Lantos, the gym teacher at Edriel's school, finds Alfred's sudden friendliness towards Edriel suspicious. At first, he is only looking out for his student's best interest, but the more he investigates Alfred, the more obsessed he becomes.

Alfred has recently transfered to Edriel's school. He quickly becomes popular, bringing an influx of new students to Eliam's history club, and challenging Edriel's status as club president. Eliam names them co-presidents and Alfred offers Edriel his friendship, but his exact intentions are still less than clear.

Ilse almost never speaks, and spends most of her time at Alfred's side. People wonder whether they are dating, though Ilse behaves more like Alfred's bodyguard or attack dog than girlfriend.

Lucia secretly likes Edriel, partly because he is shy and lonely like her. She has become suspicious, noticing that Edriel and Eliam seem to disappear together for extended periods of time.
Thiago had no reason to wonder whether he was gay until his supposed friend Sid goaded him into messing around while they were playing video games. He is not happy at all about the turn of events.
Fabien is friends with Sid and Thiago. He realizes that something awkward has happened between his friends.