Sebastian is a younger son of a knightly family, the Fitz Gabriels, who claim descent from a mighty angelic servant of Heironius. The family worships that god devoutly to this day and has produced a number of notable warriors, clerics and other do-gooding types. Though his oldest brother Michael is set to inherit the manor and lands, Sebastian is a source of pride to his family as he is a promising young brother of the Right Holy and Valorous Order of the Knights of Thunder.
Sebastian pretty much grew up in a seminary, so he is rather given to moralising, pious ejaculations, etc. He’s got a sort of school-prefect mentality exacerbated by the ethos of valour so, although he’s obedient to clear authority figures, among those he sees as his peers he tends to put himself forward and take things on personally, and he orders around those he sees as inferiors.
Other than that the usual lawful-good gig; honours word, tells it like it is, gives alms, protects the weak, etc. Is not actually teetotal but disapproves of drunkenness, won’t gamble, etc. Some of his naivety has worn off in the last year or so, as he has mixed with non-brothers and faced evil. Tends to regard most of the world with some suspicion so not hopelessly gullible but does tend to be taken in by a decent show of respectability. And still has zero experience with women other than his own family.
Tends to use lightning imagery in his speech and refer to valour a lot. For example, blast, strike, etc. are curses (eg blast you, strike it). Blasted, bolt-struck, craven, white-faced are epithets and insults. By thunder is a favourite exclamation. Brave is a common term of commendation. His language is rather archaic – thees and thous, ‘tis, have ado, and all that. If churchy Latin can stand in for celestial he’ll probably sprinkle in a few tags, prayerlets, etc.