A 'favorite' canon scene isn't the right word. No, but it's a striking and important one, important enough that I have a variation similar in my one story.
for context, Yui (the MC) passes out while caring for Reiji who is in a coma after a fatal wound. Somehow, the two meet in their dreams, and Reiji is a child in there, and he is as he often was in his youth, alone.
With total knowing, complete understanding, he explains it simply to Yui. Everyone is scared of him. They were scared of him as a child. A CHILD.
Now, I often say how he's my autistic vampire, and here it is again, because he shows it in many moments, including his youth. The other vampires are scared and unsure what to do with him this boy who stared too much, who spoke 'weirdly', who was particular and knew too much about medicine and poisons and plants. Who had little filter to his words.
A, the brilliant young Vampir he was, knew he was 'weird', and image became extremely important to him. In large part, because of his loneliness (brief mention of his mother's neglect to over burden his older brother Shuu).
Se etiquette becomes even MORE important. And we see his character, from the very beginning of the series, is particular. Part of this is genuinely hum feeling this is the proper way, and yet. This is how you act. This is how you hold yourself. He is constantly masking and presenting the ideal him in a lingering fear that he will be all alone, something that he can't take any more of.
Early in the franchise (first Drama CD perhaps?) he is shown as a dichotomy of gentleman and monster, and in a way, the gentleman is a mask, and the monster is simply an extreme, a reaction to gorcing himself into such a mould.
Because really, deep down, he's still that young boy who loved what he was, was genuinely himself, and that was good enough if only others around him could see it.