Senior and his son live on a fairly intimidating private island, somewhere off the coast of Europe. Kim first encountered them when Junior's giant tanning lamp was causing rolling blackouts across Western Europe. Upon going to their island and realizing that Senior didn't mean any harm, she gave him a lecture about energy conservation and let him be.
Unfortunately, during that same encounter, Ron commented that several features of Senior's island bore strong resemblance to a villainous lair, and gave him more suggestions for making his home more villain-worthy (Spinning tops of doom, self-activating lasers, speedboat for quick escapes, etc.). Senior originally dismissed Ron's suggestions, saying that they sounded too sinister, but upon reflection, he decided that he needed a new hobby.
Senior has gray hair that is combed back from his forehead and white at the bottom, tan skin, pale blue eyes, and a thick unibrow. He seems to have brown or black eyelashes. He usually wears a burgundy suit coat with a white cravat and some sort of blue shirt underneath, khaki slacks, and brown shoes. He has a tattoo of three S's on the back of his neck.
Senior is always extremely polite, even while he's trying to kill Kim. This goes beyond sarcastic or insincere flattery; it seems like he's just a genuinely nice person. He's also very involved in his son's life, to the point where he has trouble letting Junior plan a crime by himself. He also seems obsessed with traditional villainy, telling Junior at one point not to finish off Kim Possible so that she could have the chance to do something heroic.
He treats his villainous acts like a hobby, nothing to upset over if his fiendish plots are foiled, although his reasons and actions are often wicked and/or selfish, whether or not he intends any true ill will.
Senior does not work with other villains (other than his son, of course) and is less than pleased with Junior's occasional partnership with Shego. Senior's only known family is his son, Señor Senior Junior. They appear to be fairly close, although Junior somewhat resents his father's focus on traditional villainy and would rather use their vast fortune to become a pop star.
He formerly belonged to a billionaire's club (He was ejected for fighting with an American cheerleader and losing. It seemed a bit "too weak-like"), and regularly has the four other richest people in the world (including Pop Pop Porter and Martin Smarty) over to play cards.
- Senior's first appearance is in the episode "The New Ron". His voice was originally provided by veteran Mexican actor Ricardo Montalban, but he's later voiced by Earl Boen after Montalban passed away.
- As noted by Shego, Senior does not have any servants.
- Senior is one of five richest people on the Earth.
- Weakness of the Senior are his passion for "villain traditions" and his extremely complex plans.
- Neither father nor son make an appearance in the Shego-controlled world seen in "A Stitch in Time". A possible explanation is that Señor Senior Senior, by this time in the future, has died of old age or abandoned his villainous ideas. Another possibility is that the Seniors have pledged their loyalty to Shego and serve as her underlings in some capacity.
- It's never revealed who the mother of his son is.
- While he doesn't appear in the live action film, he is mentioned by Drakken as one of the villains mentioned to have cooler lairs than he does on "Villainstagram".