Set photos taken near the start of production had revealed a figure who appeared to be Kryptonite-powered, and nearly everyone had assumed it would turn out to be Metallo -- the cyborg with a Kryptonite heart.
His origins have been slightly different through various reboots and redesigns of the character over the years. On Supergirl, here's how he's described:
When international assassin John Corben is badly injured after taking on both Supergirl (Melissa Benoist) and Superman (Tyler Hoechlin), he is reborn by a shadow organization into the Kryptonite-powered supervillain Metallo.
It's likely safe to assume that the shadow organization in question is Cadmus, best known as a genetic-engineering project in the comics but apparently in the TV universe something more sinister still. Showrunners have said that the first half of the season will be largely spent dealing with Cadmus, who Season One revealed have been holding onto Supergirl's adoptive father Jeremiah Danvers (Dean Cain) for years.
Jeremiah had been previously thought to be dead, until a Cadmus agent inadvertently revealed to J'Onn J'Onzz (David Harewood) that he was still alive and in custody. For the last couple of episodes of Season Two, J'Onn and Alex Danvers (Chyler Leigh) were searching for more information about Cadmus and Jeremiah.
Metallo has appeared in various forms in a number of Superman media, including in live action on Superboy, Smallville, and Lois & Clark: The New Adventures of Superman.
Created in 1959, Metallo has become a staple of Superman's rogues gallery, as recognizable as characters like The Parasite and Mongul. He was, in fact, the foe Superman faced in the first issue of John Byrne's 1987 post-Crisis on Infinite Earths revamp.
That in the Byrne era, he was provided with Kryptonite and more or less enslaved by Lex Luthor raises some interesting questions about him joining the show at the same time as Luthor's sister Lena (Katie McGrath).
As the show heads to The CW for its second season, it's pulling out all the stops, having cast a number of new characters, many of whom are familiar to longtime DC Comics fans. Among the newcomers are Hoechlin as Superman, Lynda Carter as the as-yet-unnamed President of the United States, Chris Wood as longtime Legion of Super-Heroes member Mon-El, Sharon Leal as Miss Martian, McGrath as Lena Luthor, Ian Gomez as Snapper Carr, and Floriana Lima as Maggie Sawyer.
Supergirl will air Monday nights at 8 p.m. ET/PT on The CW starting in October.