Unquestionably, Luna Lovegood
We only discover her from volume 5, where a chapter bears her name. Although she initially appears to be a really weird and wacky character, Luna will gradually gain importance over the pages. And I really fell in love with this character.
She will very easily integrate the main trio, including a strong friendship with Harry, because she too can see the Thestrals. This event will bring them closer together, and under her air of a perched girl, we will end up finding Luna as a real charismatic and unique character in the HP series. Moreover, just like Harry, she is initially presented as a fairly solitary character, apart.
But his optimism and tenacity make him a character of quite incredible strength. I also find Luna has a certain innocence and candor, without being naive.
Anyway, I love Luna. And just rereading my answer makes me want to get back into the books