In solo stories in anthologies Conan the Barbarian, (Marvel Comics) (1974–1991). 1.Issue 23, "Swords in the Night", Roy Thomas (w), Barry Smith (a), John Buscema/Akins/Stone (i). (First appearance in a Conan comic publication)
Check your facts Kuro Neko. "Red Sonja, the She-Devil with a Sword, is a fictional character, a high fantasy sword and sorcery heroine created by Roy Thomas and Barry Windsor-Smith, and loosely based on Red Sonya of Rogatino in Robert E. Howard's 1934 short story "The Shadow of the Vulture". She first appeared in the Marvel Comics book Conan the Barbarian #23 (February 1973)."
Well obviously Red Sonja is not Conan, but the Marvel character is from the same world as Conan. Sure she got her own comic later on but I wouldn't say it's wrong to tag Red Sonja with Conan the Barbarian.
Difference with Batman vs Superman is that both of those characters started in their own books/worlds. Only later were their stories/worlds mixed. Sonja started in Conan.
It's more like if someone posted a set with Robin from the Robin comic and tagged it with Batman. Sure Robin has his own comic book but he started in Batman.
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