Peacemaker Season 2 released a promo for the upcoming episodes right after the second episode streamed on HBO Max. In that promo, we hear Rick Flag Sr. saying that he wants to go to the Belle Reve Prison to visit a mysterious character. We don't see the character in question, but many fans are freaking out because the third episode might feature a cameo by Lex Luthor.
However, a lot of fans are missing another Superman character: Sydney Happersen. For the uninitiated, Sydney Happersen is one of several employees in LuthorCorp in the DCU movie, which was released in July. He was also confirmed to appear in this HBO Max series, and the Peacemaker Season 2 red band trailer confirms Sydney's appearance.
The mysterious Belle Reve Prison inmate that Rick Flag Sr. meets in Peacemaker Season 2 might be Sydney Happersen:

We don't see what happens to Sydney Happersen in the last act of Superman, but considering he was one of Luthor's co-conspirators, it is likely that he was imprisoned in Belle Reve along with his boss. The Peacemaker Season 2 promo is intentionally keeping the inmate a secret so that fans will keep the show in conversation throughout the week.
But given that we see Stephen Blackehart's Sydney Happersen in the red band trailer, it is highly likely that Flag Sr. enlists the former's help in identifying Peacemaker's dimension-hopping technology. But it should be noted that we still don't know the prison inmate whom Rick Flag Sr. wants to meet. The mysterious character might very well be Lex Luthor.
James Gunn has also teased cameos for the last three episodes:
It is known that select members of the press and influencers were given the first five episodes of Peacemaker Season 2. When the filmmaker was asked about the last three episodes, Gunn responded that not even his publicist knows what happens in the last three episodes of this season. When Brandon Davis asked Gunn the reason behind this secrecy, Gunn answered:
"It could mean they're terrible [laughs]. That is one option. Or it could mean that there's a lot of major twists and turns in the last three episodes. And even though I'm sending the first five to a select few, things still get out as you know. Things got out already. You know, people who want to search can be spoiled on purpose, but I really don't want what's in those last three episodes to be spoiled. And so I'm keeping them close to the chest. I won't even let my publicist see them. That's the god's honest truth"
Then, Gunn further added:
"I will also say, six and eight may be my favorite things I've done, like, of all time. It's like some of my favorite stuff ever."
These words have led many fans to theorize several possible cameos in the last three episodes of the second season.
Who, according to you, is the mysterious prison inmate mentioned in the promo? Peacemaker Season 2 Episode 3 will stream on HBO Max on Thursday, September 4, 2025.