Bowser Junior: Who is he? He isn't a Koopa Kid. Has he ever been explained? Was he adopted? Was he Baby Bowser at some point?

Dry Bowser: Mega Bowser and GigaBowser I know about, I know about dry bones too but where did Dry Bowser come from?

Wario and Waliugi: Did Nintendo just make these two to personify Sega lovers? Fat, overweight, fart-joke loving Mario haters? Are they the original hardcore twins?

Baby Mario and Luigi: Are babalities a sign of desperation in the gaming industry? Mortal Kombat was getting stale so probably needed them but Mario.......?

If anyone can help me out I'll explain the secret truth about Birdo.

Bowser Jr: Probably a Bowser Kid too, as he was seen with the Koopalings in NSMBWii.
Dry Bowser: Bowser after being burned alive.
Wario and Waluigi: Antagonists, Wario and Waluigi should technically be independent of Mario/Luigi
B. Mario & Luigi: Probably they met with Mario & Luigi with E. Gladd time machine, and come back for sports and stuff.
Birdo: a guy.

Bowser jr. came from Super Mario Sunshine. He wasn't one of the eight original Koopalings but he's a legit Koopa kid. He was probably born late.

Wario came from Super Mario Land 2. He was like an evil Mario. There was a big backstory about him and Mario being friends as kids from Nintendo Power back in the day. I can't remember any of it.

Waluigi is like opposite Luigi or something. I read somewhere that the "Wa" in Wario and Waluigi means evil or negative or something. My first time seeing Waluigi was in Mario Party or Mario Golf or something.

Dry Bowser is the result of New Super Mario Bros. Mario dumped Bowser in lava in the beginning of the game. It burned all of his skin off.

Baby Mario is Mario from Yoshi's island. He got separated from baby Luigi when the stork was delivering them to their parents. The baby versions of Mario and Luigi started working with adult versions of the Mushroom Kingdom characters in Mario Kart Double Dash. Later, in Superstar Saga 2 for the DS (I never played that one, though).

birdo is a transvestite i think which means man became a woman i think

Some sort of time travel deal is involved with the babies, except for Bowser Jr, who has a magic paintbrush given to him by Elvin Gadd, costar of Luigi's Mansion