Bowser

King Bowser Koopa is the final boss in most mario games. He is the main leader of the Koopa Troop (which includes the Koopalings, Bowser Jr., Dry Bowser, Dry Bones, Koopa Troopa). For some reason, he is the one who kipnaps Peach the most often. He is arch-nemesis with Mario (the lover of Peach). He wants to conquer the Mushroom Kingdom and after that, he wants to rule the world. Since this exciting character exsisted, Bowser is in most mario games. He is also a playable character in some games.

Mario
As I told you, Mario is arch-nemesis with Bowser. Mario is the person who often wants to save Peach from Bowser. Mario is the person that makes Bowser extremely mad. Bowser has a bit of respect for Mario, especially in Super Mario Sluggers when Bowser saved Mario from a Bullet Bill sent by Wario and Waluigi. Unfortunately, Bowser saved him so that he could destroy Mario himself.

Princess Peach
Bowser hates every resident in the Mushroom Kingdom, except for Princess Peach. We think that Bowser kipnaps Peach to get Mario into trouble but, we found out that he loves Peach. Bowser does harm her but, he doesnt go as far as killing her or treating her kindly. Bowser even forced her to watch Mario almost getting killed (Mario is her true love) and he was tied to a rope. Peach is able to recognize the soft side and sometimes, she kisses him to agree to free Mario, and Bowser mostly agrees. Bowser does not care about anything else than Princess Peach, his chilren and his army.

Children
Bowser has eight children in total. He cares about his children deeply, as for their loyalty to the Koopa Troop. Sometimes, he uses them as slaves. He cares about Bowser Jr. the most of them all. He lets Bowser Jr. go on a higher level than the other seven siblings.

Luigi
Bowser briefly forgets about Luigi and calls him Green Statche. When Bowser captured Mario in Mario is Missing, he thought he was unstoppable until the time when Luigi came and stoped him. That's when Bowser found the threat level in Luigi. Bowser has captured Luigi in Super Mario Galaxy and in Super Princess Peach. Lugi is still in the group which Bowser hates. The other two people are Mario and Yoshi.

Yoshi
Bowser and Yoshi has met each other a lot of times since Bowser was a baby. Yoshi saved Mario, Luigi   and Wario  once. Bowser thinks that he is one of the biggest enemies he has.

Wario
Though Mario is enemies with Wario, Wario doesn't lilke Bowser either. In several games such as Super Mario 64 DS, Wario helps Mario and Lugi defeat Bowser.

Koopa Troop
Bowser cares about his army, only when they fail him, he cares a little about them. His troop respects Bowser more than fear. His troop does not say mean things about Bowser, even when he's not there.