Bowser wasn’t always the way he is now, years and years ago he was known as Prince Barclay, a temperamental man who desired total control. In this time he was betrothed to Princess Toadstool, much to her dismay. She rejected him for a later suitor. In his blind fury he turned to his dark magic and sent monsters to retrieve her (Goombas, Koopa Troopas, etc.). In an attempt to keep her to himself he manipulated her to sleep with him, this resulted in his first seven children, Princess Toadstool was missing for a year while she was hidden away during the pregnancy.
The identity of the kids' mother was kept under wraps for blackmail purposes. This was when Mario was first sent to look for her. In an attempt to become powerful Barclay cast spells on himself, unaware of what they would do. He and his children were turned into monsters and assumed new names. Barclay, now Bowser, vows to retrieve his love and keep her.
Five years later Bowser convinced the princess to fool around, and later she gave birth to Bowser Jr. Once again hidden for a year to care for the newborn, she did bond with her other children.
Toadstool was her birth name but she was always called Peaches since itit w her favorite treat growing up. Her father called her this often and she later took on the name Peach in loving memory of him.
Mario wasn’t as famed in the Mushroom kingdom as he is now, until the news spread of his events in Sarasaland to save Princess Daisy, when Peach learned of him she felt she had to meet him, upon meeting him and spending some time together they became intimate, calling off her engagement.
Bowsette (Placeholder name til I can find something more suitable) exists in an alternate branch of the Mario world where Bowser finds a special power up that grants transformations.
He swaps between his normal form and this new princess form in his Chambers before showing it off to test limits and get used to it.
In a way the Bowsette form gives him a refresher, having been beaten by Mario many timeszhaving been rejected by the woman he loved countless times, he felt deflated. In this new form, he used it to form one last chance to get Peach.
In his Bowser form he faked the death of Barclay, making it look like one final monster got to the prince. Bowser cannot speak, the one time he does he just focused on time with his son and Peach (Super Mario Sunshine). Bowsette claimed to be a lost daughter of Bowser, and used this form to invade Mushroom Kingdom. When she had Peach cornered, she just explained what happened to Barclay all those years ago. And insist Peach be with her. Peach isntsure what to think or if this was a trick but the eyes tell all.
Bowsette is a bit of a tsundere when it comes to Peach but enjoys the bubbly princess’ company nonetheless. Bowsette later returns to find her home kingdom still thriving via the royal advisor’s family. Bowsette took the thrown for her family. She did attempt to form a relationship with Peach that ended in a politically driven marriage, well, on the surface. In private Bowsette loves the shit out of her little bubbly blonde wife.
The Sonic and Mario universes are extremely close, boarding on each other with portals linking them together with ease, giving both inhabitants time to play sports together. Mario and Sonic have both a healthy rivalry and almost brotherly bond.