Mortimer McMire, also known as The Grand Intellect (and to his annoyance, also as the Gannalech), is Commander Keen's ultimate nemesis, an evil human prodigy with the IQ of 315 who will not rest until he has destroyed the whole Universe.
Not much is known about his origins or family, except for his sister Molly. Mortimer seems to be about the same age as Keen (and was probably his classmate), so he should be around 8 years old.
Mortimer believes himself to be of the highest intelligence ever and has decided that nothing else deserves to live. His ultimate goal is to destroy everything, because everything is so stupid. It is not known how he is going to survive if he destroys the whole Universe, or whether he is even wanting to survive or just end it all and himself along with it.
Like Keen, Mortimer's appearance has changed throughout each game he appears in. Generally, Mortimer resembles Keen a lot, as they both wear football helmets and have pale skin. In fact, Mortimer looks quite like an evil version of Keen.
In Keen 3, the Android Dummy, who may be designed to look just like Mortimer himself, is seen with a Chicago Bears helmet, black eyebrows, and either black overalls or a black tank top. In the ending of Keen 5, Mortimer's helmet is black with two red Ms on it (standing for his name). In Keen GBC, he wears a black helmet with a red stripe, black shirt, black pants, and red shoes. He also has shades, a red belt, and lighter skin than Keen's.
Mortimer's appearance varies in Keen mods and fangames, although he is usually depicted with his helmet from Keen 5, a red shirt, either blue or black pants, and red shoes.
Tom Hall has confirmed that Mortimer wears a Chicago Bears helmet (with the logo absent to prevent copyright problems, although he said that Mortimer could have a GI logo in SGA), black shirt, blue jeans, black eyebrows, and black shoes. 
Rule over the Vorticons
The exact time and means of McMire's seizure of power over Vorticon VI is unknown. In fact, it cannot even be said why McMire chose the Vorticons to be his slaves, although it is probably related to their sturdy physiology. In any case, what is known is that, by the time Commander Keen was stranded on Mars, Mortimer had the entire species under his absolute control through a mysterious and evil "mind-belt" technology. He adopted the title "The Grand Intellect", which seems to be the alias he uses when dealing with alien species (similar to Billy Blaze's "Commander Keen" alias).
It is clear that McMire's dictatorship led to a great disruption of Vorticon life. In particular, Vorticon VI became one of the most militaristic worlds in the galaxy, as part of McMire's plan to obliterate Earth and, more fundamentally, as a mere pawn in his greater schemes. All male Vorticons were trained from a young age to fight to some extent. Most became grunts, and either labored for the Intellect all over the home planet or, worse, were sent to serve on the Vorticon Mothership. Such insidious orders as the Vorticon Elite and the VortiNinjas were created, forming the galaxy's deadliest fighting force. Even the Vorticon Women's ability to spit fire was used for destructive purposes.
The absolute rule of the Intellect began to unravel with Commander Keen's discovery of the Vorticon Mothership. After escaping Mars, Keen boarded the heavily guarded mothership and eliminated the deadly Tantalus Rays. The incapacitation of the Mothership, though a major setback, did not end McMire's rule and, in fact, probably only made it fiercer. As Commander Keen arrived on Vorticon VI, the Vorticons were essentially ordered to carry out a vendetta against Keen.
It was not enough, however, and Keen was able to make it to the Grand Intellect's castle. It was here that Keen discovered the Grand Intellect's true identity. He fought McMire in his hideous Mangling Machine, and brought an end to his tyrannical rule. Though the end of the Vorticons' oppression, it was by no means the end of Mortimer McMire.
At first Keen had no idea that Mortimer was plotting something — not until he discovered the Vorticons actually were not hostile but were brain-controlled by Mortimer McMire (see Keen 3 Story). By destroying the Mangling Machine, Keen believed he had gotten rid of Mortimer. One day, as he was listening to his Photachyon Transceiver, he picked up a distorted signal. He managed to hear something about the Shikadi planning to destroy the Galaxy, and quickly set off to Gnosticus IV to learn more about them (see Keen 4 Story). The name Gannalech was also mentioned in the message, but Keen did not pay much attention to it at the time.
After stopping the Shikadi's plan he finds a message on the Omegamatic, The Mortimer Letter (see Keen 5 Story). In the letter, it is revealed that Keen did in fact not destroy Mortimer, the Mangling Machine was only operated by an Android Dummy, and that Mortimer was behind the whole Shikadi plot. "Gannalech" was what the Shikadi called him, being the way they pronounced "Grand Intellect". The Shikadi plot was simply a diversion to stop Keen from discovering Mortimer's one true goal: he intends to blow up the whole universe, not just the galaxy. However, for reasons unknown, Keen decided to decipher the message later — perhaps he was in a hurry to get home, as at the end of Keen 4, he says he hopes "it does not rain on my folks."
Because of this, it seems that he forgot all about the letter, and did not learn that Mortimer was still alive until much later, when he had to save his baby-sitter from the Bloogs after she was kidnapped (which was also a diversion — see Keen 6 Story). After the rescue, when he discovers that Mortimer is alive and well, he learns about Mortimer's plans to destroy the entire universe. (Given his surprise at discovering Mortimer is still alive, it seems he really had not read the letter.)
Keen was to confront Mortimer in the final battle for the Universe, but it never happened, as the final game in the series, The Universe Is Toast, was iced. However, many fans still hold out hope that it will be made one day. (Tom Hall, the lead designer for the original games, has made it clear that he would make this game without hesitation if creative control was again returned to him.)
Appearance in Keen GBC
Keen GBC is the only official game where Mortimer appears in person. He looks basically the same as Keen, except for wearing a black and red outfit as well as shades. Unlike in Keen 5, his helmet features a red stripe.
The game manual describes Mortimer as the following:
Commander Keen's arch-nemesis, school rival, and all-around evildoer. He's masterminded your entire adventure and wants to see the Universe destroyed! You'll never catch him, but he's out there! Waiting to see you fail!
- Keen 2 – referred to (as "Grand Intellect")
- Keen 3 – as the end boss (first time refered by his real name)
- Keen Dreams – referred to in the story
- Keen 4 – referred to (as "Gannalech")
- Keen 5 – hinted to with a Standard Galactic Alphabet letter and his helmet
- Keen 6 – mentioned by sister
- Keen GBC – various appearances