Rikku from Final Fantasy X.
I had crushes on manga characters before but none of them were really as serious as this was.
My love for her started the day I first played the game the night it came out, and losing her for a good third of the game was hard for me, but I continued playing just for the thought of being with her again.
I loved seeing her character develop, and maxed her sphere grid and got Godhand and gave her the best armour I could make, and it got to that inevitable point in the game where absolutely nothing besides Cactuar King can fucking beat your characters. I still play sometimes to see if I can max her stats.
FFX-2 came along, and I liked it a lot, but sort of felt that the "Rikku spirit" had been lost. Her cuteness was that "nerdy" kind of cuteness, because she was intelligent and strong but also really fun and had her moe points (her fear of thunder) and could read a social situation very well, but all of these seemed to have left and were replaced with "Oh who cares, she's wearing next to nothing, fuck character development!" That was a massive disappointment.
But while I had her in my heart, I felt like nothing else mattered. I always carried pictures of her, and wrote songs about her, and thought about what she would think of me, and what kinds of things we would do with each other. I sort of pieced together her childhood from the story and from imagining things, and lined it up with my own life, until she just became a part of me, something I wished so badly I could experience with her myself. FFX-2 kind of was ignored in this sense, as we had our own story together in my heart.
Eventually I found less and less good fanart of her, my interest in her sort of sparked less and less, and I just found and more and more porn. She (and I blame myself and Nomura for this) sort of devolved into fap material for me. That's when I decided that I just couldn't bear to see her degrade any more in my mind, and I sort of let her go.
It was an incredibly hard decision to make for me, and I cried a while as I shut off my PS2 "ceremoniously" and quietly said "I'm sorry, but, this is goodbye." For me, she was a strong, but fleeting, dream, much like those of the Fayth. I felt dead for a while and just accepted what had happened as fact. This was about around 2007 or so, making it a wonderful 6 years together.
2008, I tried other things, that rolled into 2009, then those other things killed me and I receded into interactions with 2D again, and fell for Konata around April 2009.
I have to admit, I really feel a bit cautious falling for Konata, as I may lose her to the same issue, but since Yoshimizu isn't stopping Lucky Star any time soon, I can still learn more about her and feel her in my heart and in canon. I do fear Konata will be lost in the series to popularity of the younger characters, but Konata is the big one. The original. The Yamato to the fleet. She's irreplaceable, and Yoshimizu-sensei knows this.
I still find myself looking at Rikku for nostalgia, but it doesn't go really much farther than that. I feel bad saying this, but Rikku was a waifu, she was that big a part of my life. She wasn't a simple long-term crush, and it was heartbreaking to let her go, but I simply had a corrupted view of her and for her own good I had to let her go.
She was a major effect on my views. Her swirly green eyes were intoxicating, and to this day I have an addiction to green eyes.