Third Summers brother

The third Summers brother is a largely speculative plot point launched by 1993's X-Men (vol. 2) #23. In that issue Mister Sinister has a conversation with Cyclops and casually mentions Cyclops' brothers. Cyclops notes that it is odd to talk about his brothers in the plural, since as far as he knows he has just one brother (Alex Summers, also known as Havok).

While Sinister himself immediately claimed that it was just a common mistake of his, the speculation about the mysterious third Summers brother ran rampant among fandom. Gambit and Adam X were both believed to be the missing brother at some point.

The second X-Men: The End series, arguably out of continuity, established that Gambit was the third brother.

The 2006 series X-Men: Deadly Genesis established however that the third Summers brother did indeed exist and was the previously unknown character known as Vulcan.