Flatline is a new character being introduced to the DC Universe by Joshua Williamson and Gleb Melnikov in their new Robin series starring Damian Wayne. James Tynion IV confirmed as much in his newsletter, promising an eventual Punchline Vs Flatline and saying "And we got the first glimpse of the new character, Flatline, who will be debuting in the first issue of the book."
But it turns out, Robin #1 is not the first appearance of Flatline in a DC comic book. Detective Comics #1034 out yesterday did plenty for Gotham, Mayor Nakano – and the Green Arrow – But in the back-up Robin strip, there is a glimpse of the other members of the Legion of Lazarus.
Which makes Detective Comics #1034 the first cameo appearance of Flatline – for comic book collectors for whom this kind of thing really, really matters.
DETECTIVE COMICS #1034
(W) Mariko Tamaki, Joshua Williamson (A) Dan Mora, Gleb Melnikov (CA) Dan Mora
DC Future State may be over—but the present looks tense! With the loss of his fortune and manor, the election of Mayor Nakano, and the growing anti-vigilante sentiment in Gotham, Bruce Wayne must rethink how to be Batman…or risk being left behind by his own city. To make matters worse, a catastrophic crime wave has taken hold of the city, culminating in a murder mystery that hits close to home—the suspects mount, the clues multiply, and the trail of bodies hasn't ended yet! Eisner Award-winning writer Mariko Tamaki and superstar artist Dan Mora begin an exciting, surprising, and death-defying new chapter of Detective Comics. Plus, in the second and concluding chapter of "Demon or Detective," Damian Wayne meets a startling new faction from his family's checkered past…but what is the League of Lazarus? In-Store: 3/23/2021 Retail: $4.99
(W) Joshua Williamson (A/CA) Gleb Melnikov
After learning of the deadly League of Lazarus tournament, Damian Wayne has a new mission: winning the tournament and prove he is the greatest fighter in the DC Universe! But first he must find the secret island where it's all going down! This brand-new solo Robin series will force Damian Wayne to find his own path away from both sides of his family! New mysteries! New supporting cast! First appearances of new characters! And lots of fights! In-Store: 4/27/2021 $3.99