That letter is "Đ", 7th letter in Vietnamese alphabet, it's equivalent with letter "D" in English one.
I usually write his name "Dan" instead of "Đan" (accurate form) when chatting with non-vietnamese friends. But well, now i think i should write his name correctly, you can write "Dan" as usual, it's okay.
Ah, i love him so much >_<
Tools: Watercolors.Nghiêm Đan © meRelated drawing: