Sebastian or "Bash" is Prince Francis' half-brother and bastard son of his father King Henry II. While the two have different mothers, they do share the same type of heart I think. I really liked his character and never saw him in a negative light most of the time.
In reality Bash never existed, but his mother did. Diane de Poitiers was indeed King Henry II's true love (but not a legal wife) and she was actually about two decades older than the king. In Reign it's presumed she's somewhat younger than Henry but certainly much older than Kenna. It is shown that Henry prefer Diane and Bash to Queen Catherine and Prince Francis on a personal level.
In real life she was once the most powerful woman in France (Catherine didn't have much power until after Henry died) but in the show she helps in ways to get her son the throne.
Which he doesn't really want, but he does want Mary, so he agrees to a point. This whole story-line was odd because the House of Valois was on the verge of extinction and the Catholic church would not agree to annul Henry's marriage with many heirs to give a bastard a chance.
This blows up in Bash's face, and he is forced to marry Kenna after King Henry goes crazy.
But they stay married and happy after the king's demise.
Well until King Antoine of Navarre (who wanted revenge on Bash over the death of his brother during a war) destroyed their marriage for the most part.
After his failed marriage and Francis' death, he stays in the same role to protect his other half-brother King Charles IX.
But he goes with Mary when she is forced to return "home" to Scotland. After some trouble, they separate forever. I thought he would have a bigger role in the Scotland side once I saw him go along with Mary on the ship, but it just ended. Rather odd I think.