Bowser

Bowser is a turtle (dragon? ...is that ever explicitly stated or...) who is usually Mario's arch-nemesis. He is the leader and king of the Koopa Troop army, likes to capture Princess Peach or try to conquer the world, although his plans almost always fail, and he often needs the help of SuperMarioGlitchy3. Bowser has been shown to have used other methods of destroying Mario as well, such as creating 4 Mario robots & giving people nightmares with a crystal stone.

Mario
Mario and Bowser enemies in the official Mario games. In SMG4's bloopers, they are still enemies most of the time, but the two sometimes cooperate when they have a common goal, like hating Peach or cooking (although the latter ended when Bowser tried to make a Ztar, which turned into a hamburger instead, causing SuperMarioGlitchy3 to go make a Ztar himself).

SuperMarioGlitchy3
Bowser and SMG3 are the main antagonists of many bloopers, often hatching plans to kidnap Peach or kill Mario and SMG4. Being the evil idiots they are, they often disagree and fight with each other. This inability to cooperate has led to many failed plans. The two only occasionally interacted until the blooper Two evil friends, where they were forced to become partners by Bowser's boss.

Princess Peach
Bowser's most common evil plan is kidnapping Princess Peach. He has a secret crush on her, which is the basis of the plot of "Bowser Cyrus - Wrecking Ball".

Trivia

 * He can successfully copy Peach's voice.
 * He hates SMG3 unless it happens that he lives with him.
 * He's rarely the actual antagonist in bloopers, contrary to his true nature in Super Mario 64 and the Mario series in general.
 * He can fly.
 * He loves Justin Bieber, just like Toad and Yoshi, but in the blooper "Mario's Guide to Defeating Bowser 2.0", when Justin Bieber's song is played in the radio, Bowser didn't like his music. He shouts "I'm trying D: AHHHH MY EARSS!EFRW."
 * In "The Adventures of Mario and Luigi - Ep. 4", he was in love with Princess Daisy.
 * Even though in Super Mario 64 he hates Mario, in SMG4's bloopers, Bowser likes Mario.
 * He has a JB cutout, which he makes love to as shown in Ssenmodnar 6.64(100k special Qna Edition).
 * He's in love with Peach, as seen in "Bowser Cyrus - Wrecking Ball".
 * According to "Cooking with Bowser and Mario 2!", he loves melon.