Mewtwo
Mewtwo is an excellent addition, no matter what kind of team is being made. In addition to high stats, his move set contains Psychic and Fighting attacks. Due to this, no other Pokemon can be completely immune or resistant to his attacks. His only weaknesses as a Psychic type are to Ghost, Dark and Bug attacks. All of these are fairly rare. He provides a powerful all around threat and makes a good fall back choice if Charizard is injured, regardless of the opponent.
Zapados
Zapados is good choice for being included in a Charizard centered team. His Electric attacks allow him to easily neutralize Water Pokemon, one of Charizard's main weaknesses. He also maintains similarly high stats in every category. His Drill Peck attack is very effective against Grass and Bug Pokemon. This can provide an alternative option for dealing with these types without using up Charizard's Fire type attacks.
Aggron
Aggron is a Steel and Rock type Pokemon and has one of the highest physical defense stats in "Pokemon: Fire Red." He can easily absorb damage from most opponents. Only Fighting, Ground and Water Pokemon can do extra damage to him. Charizard is immune to Ground and resistant to Fighting attacks, so you would not normally need to use Aggron against those types. He is also immune to one-hit knock-out abilities, allowing him to safely fight opponents that have Fissure, Guillotine, Horn Drill or Sheer Cold. He also gets Double Edge, a powerful attack that normally damages the user. Aggron does not take damage when he uses it. This gives him the ability to put out a decent amount of damage while he is absorbing damage in Charizard's place.
Kyogre
Kyogre is a Water type Pokemon with higher than average stats. This Pokemon is used to nullify Rock type Pokemon, one of Charizard's own weaknesses. Water attacks are also very effective against Fire and Ground Pokemon. These types do not pose a particularly major threat to Charizard, but Kyogre can take out Pokemon with these types more efficiently. This prevents Charizard depleting his attacks
unnecessarily.
Golem
Golem provides a method of dealing with Electric type Pokemon, which deal extra damage to Charizard. The only reason to use this Pokemon is when your opponent uses an Electric Pokemon. He is completely immune to Electric attacks. His Ground attacks are very effective against Electric types and, similar to Aggron, he can use Double Edge without damaging him. He is not that useful against most other Pokemon due to his massive number of Weaknesses. His only role is to safely plug up Charizard's Electric type weakness.