Birdo.
Birdo and Bowser had a brief relationship prior to the beginning of Super Mario Brothers one. Her main power is spitting out eggs...So she spit out the eggs of Bowser's first seven children. However, things were not to be and Birdo and Bowser split up on bad terms. In between Super Mario Brothers 1 and 3 these children then hatched and grew up enough to be toddlers. It was during this time that Bowser realized his one true love on the planet is Princes Toadstool and he focused all of his attention on her. Birdo, on the other hand, fell into despair, and end up staying in the world of Sub-Con for a period of time.
The lives of Birdo and Bowser would not intersect again until the time frame of Mario Tennis 64. Birdo had returned to the Mushroom Kingdom and was once again playing the singles game -- as in, dating. (But also tennis.) When Bowser saw how well she was getting on with Yoshi old feelings bubbled up to the surface and he was filled with jealousy. He made romantic overtures towards Birdo once more and they spent a week-end together. At the end of the week-end, however, it was once again apparent that they were not meant to be, and Birdo left. A few months later Birdo called Bowser and said she was pregnant.
So Bowser was left with another Koopaling egg. Inbetween Mario Tennis and Super Mario Sunshine this egg would hatch into Bowser Jr. After seeing what lazy bums his first children were he decided to take Bowser Jr. under his wing and teach him the ways of tyranny. Eventually Bowser Jr. asked Bowser who his mother was. Embarrassed about his romance with Birdo, he instead told Bowser. Jr. that his mother was the princess. Birdo is still seeing Yoshi off and on.
That leads us to today. Any questions?