Not unlike any other mounted model, a character that is mounted upon a Ridden Monster will have a split profile. In game terms, this works as follows:
Unless noted otherwise, any special rules that apply to one element (character or mount) apply to the other as well.
This model uses the Movement characteristic of the mount.
The character and mount each use their own Weapon Skill, Ballistic Skill, Strength, Initiative and Attacks characteristics, and their own weapons.
In combat, all enemy rolls To Hit are made against the character's Weapon Skill.
Impact Hits and/or Stomp Attacks use the mount's Strength.
When this model makes an Armour Save roll, it may use either the mount or the character's armour value, whichever is better.
If the character is reduced to zero Wounds, the model as a whole is removed from play.
Improved Characteristics
Ridden monsters are invariably huge and fearsome beasts, and being mounted upon one will increase the character's Toughness and/or Wounds characteristics. In such cases, this will be noted within the mount's profile.