Sir Ystin (Demon Knights)
Introduced by Grant Morrison in DC’s 2005 miniseries Seven Soldiers, Sir Ystin, a.k.a. the Shining Knight, started out as Ystina, a female admirer of Sir Galahad who disguised herself as a man to fight in the doomed battle to save Camelot. He reappeared in the current series Demon Knights, fighting evil alongside an actual demon, a Muslim alchemist, a tarot-card reader, an exiled Amazon and others.
Coming from a much earlier era, Ystin doesn’t really discuss his gender or how he feels about it, but he presents himself as, and is generally referred to as, male. Sir Ystin is perhaps the only implicitly trans characters in a current Marvel or DC series.