If I have to say partner, then I actually have 2 Pokemon that I have considered as partners.
The first one was my Milotic, Mystica. I caught her as a Feebas back at Pearl. After growing a ton of Pamtre berries, I managed to make her evolve, and she has helped me ever since. When I first played Platinum, I transfered her over to help me, plus to train her to lv 100. When I first played HG, she also helped me.
The second one was my Togekiss, Chelinka. I got her out of Professor Elm's egg. I didn't expect it to be a female, and I managed to save her and a few other Pokemon before my Jpn HG got deleted. Although... for some reason I got another female Togepi from Prof. Elm at my SS playthrough. Maybe I should conclude that female Togepi are my partners :3