We know for a fact Sett has a Noxian absentee father. It's obvious who that is.
His hair, demeanor, face, and overall features.
He's none other than Katarina's half brother.
All the evidence is blisteringly obvious, General Du Couteau is currently missing, known to be a family abandoning self important character who was last seen in the noxian invasion of Ionia, of which from Vastayan lore we also know that some Vastaya actually joined the Noxian side during their invasion, choosing to favor those who they figured would win, not to mention that there are those who hate the humans of Ionia
as they are a direct impact danger to their lifestyle, and even implied to be ability to procreate and proliferate; due to magic being necessary for their continued life.
a vastaya who has a very similar appearance to Sett's mother is also very clearly anti human and self serving. She does her own thing. So its not hard to believe someone who would marry a noxian or at least breed with them is unbelievable.
The face shape with the angular design and the cheekbones, as well as eyes, and the unmistakeable immediately noticeable
exact shade of red hair.
We also know, that according to Cass's lore he joined what was apparently an anti old guard coup de'tat attempt, along with General
sometime during or after the Noxian invasion of ionia, at which point shortly after General Du Couteau went missing; his household wife hating him as she was part of the black rose old guard to whom
was a direct opposition of.
He could not go home to a wife that would kill him, he was knee deep in enemy territory and its not unbelievable he would be otherwise deep in a romance with someone new. Who's natural born brutality and finesse for combat as a vastaya would likely excite him.
Sett; is, in my opinion, child of Du Couteau, and therefore is Katarina's half brother.
Also I know theres a lot of memes about it but No; Draven cannot be his father. Riot has said as much.
Red hair; especially such pure/brilliantly red. Is a recessive and rare trait. Brown and black hair are dominant. Its excruciatingly unlikely for anyone with hair that's not red to be their father.