Brooding, Solitary Man
A brooding, solitary, lonely man—that pretty much sums up the speaker, and a lot of Byron's protagonists. Think about it: this guy grieves silently and doesn't let anyone in on his secrets, he feels lonely and upset even when he's around company, and he clearly has spent plenty of time thinking about this old flame of his.
Well, guess what. This dude appears over and over again in Byron's poetry, especially in early works like Childe Harold's Pilgrimage and, perhaps most obviously, in Byron's closet drama Manfred, which is about a dude that made a deal with some mysterious spirits and broods in his mountain solitude about the consequences of that deal.