Damon is the brother that loved Katherine for 145 years. He was the brother determined to rescue her from the tomb under the church. Yet he's the brother whose relationship with Katherine hasn't really been told.
We know about Stefan and Katherine's first kiss, but we don't know about Damon and Katherine's. We know how Stefan and Katherine met, but we don't know how Damon met Katherine. We know about the first time Stefan had sex with Katherine, but we don't know about Damon's first time with Katherine. We know how Stefan learned that Katherine was a vampire.
That's not to say that interesting tidbits of Damon and Katherine's relationship haven't been shown. Children of the Damned had the most intriguing pieces of Damon's romance with Katherine. The episode began with Damon laying in the road pretending to be injured while Katherine attacked two men, feeding on them. With blood still dripping from her teeth, she tells Damon that's how it's done. Then she gets Damon to kiss her with the blood still all over her faith, telling him he'll have to get used to the taste of blood. Although a little squeamish, Damon kisses her passionately, blood and all.
The next flashback shows them playing in bed together and Katherine baring her fangs at Damon. He caresses her face and her fangs disappear. Later she takes him to the apothecaries were she and fellow vampire, Pearl, have a vampire meeting, right in front of Damon.
There were also some tantalizing scenes between Damon and Katherine in this year's finale, As I Lay Dying. A feverish Damon recalled a couple of interesting moments with Katherine. Damon was preparing to return to the war and Katherine made Damon promise to return quickly. Since Damon ended up becoming a deserter, did Katherine play any part in that decision? We also got to see the first time Damon consumed Katherine's blood. She refused to feed it to him and made Damon feed from her like he was a vampire.
The show really needs to tell Damon and Katherine's story. I keep thinking there's a reason they've concealed so much of it. Like maybe if it's told, we'd know that Katherine actually does love Damon and has loved him all the years he spent loving her!